It depends on the context, but for some of these:
- Some doors can be forcibly opened that way, some can't. Many dungeon doors are made of stone or metal, for example--that's been my experience with doors in 5e, for example. Wooden ones are rarely an impediment, and stone ones aren't realistically gonna yield to mere hammer blows.
- "Removal of hinges" is one of those ideas that only makes sense if the GM isn't really thinking through the design of the dungeon. If the hinges are accessible from the side of the door that's meant to keep people out, the lock is worthless and the architects were idiots, which might happen very rarely but can't be relied upon.
- Most of the spells you've mentioned are not a reliable tool for several reasons. In older-style D&D play, such as your game, IIRC spells are only received randomly and there's no guarantee you'll find them out in the world either. I'm not familiar with warp wood (sounds like a Priest spell?), but that depends on the door being wood, so same issue as the first bullet point.
- Depending on the rules involved, it's not always possible to shapeshift into something small enough to slip under the door. Certainly in Dungeon World that'd be a thing, for example, but in 3.5e, Druids cannot shapeshift into any size smaller than Tiny, which is two size categories too big to fit under most doors (you need to go past Tiny and Diminuitive down to "Fine", which is <6 in tall/long).
- Dimension door is almost surely a success if the door can then be removed afterwards, but is a pretty powerful spell to blow on just potentially getting past a door (4th level in every edition I can find--surprisingly, even in 3e which often had spells at wildly different levels depending on class). Players might rightfully question the worth of such a thing.
Sure, not all of these are going to work every time, but the options are still there to try. IME most mages try to get their mitts on
Knock as soon as they can, also in my game it doesn't have the alarm-bell feature that 5e gave it.
And if the door opens toward you, the hinges have to be on your side somewhere. If it opens away from you, they won't be, but kicking the door in becomes far more viable. The tricky ones are sliding doors.
I've once or twice even seen situations in stone caverns or buildings where a party Dwarf spent a few hours and, using the rules for Dwarven mining, mined his way around a stuck door!
The other option that every party has, and that I didn't list, is to go back to town and gather more resources and-or more people specific to the task at hand. For example, if the party doesn't have
Knock available, go back and recruit or hire a wizard that can cast it. If the party doesn't have a lock-picker, go back and recruit or hire one.
Another one - again at higher level - we did not long ago in the game I play in is the party mage polymorphed herself into an Umber Hulk and simply dug us a tunnel down from the surface in order to get into an area we knew was there but couldn't otherwise access.
Perhaps, perhaps not. Surely, though, it would be good for GMs-in-general to know and have the use of tools that can address these issues? Surely it would be beneficial to have ways to teach GMs without forcing them to make stupid mistake after stupid mistake after stupid mistake in order to finally stumble into wiser ways to approach a problem?
We probably differ in that I don't see insurmountable (or seemingly-insurmountable) obstacles as being a stupid mistake. To me, they're just part of the setting - sometimes either by good in-fiction design or bad at-table luck you're just not going to get there from here no matter what you do. So be it.
Like...that's literally what makes humans special. We can build up one lifetime's worth of knowledge...and then condense it into a few books, perhaps a small bookshelf worth of reading, which another human can then digest and understand in mere days, perhaps weeks. Developing tools and processes and procedures and guidelines and rules of thumb and (etc., etc., etc.) so that those who come after us don't have to blindly stumble in the dark until they find the way a thousand other people silently found already but failed to communicate to them.
That's one of many reasons why I talk about tools of various kinds (procedures, processes, guidelines, rules of thumb, best practices, SOPs, etc.), and why I almost always ask about them when I want to know more about a style I don't yet know. Those things are human power in action. And it's why I see TTRPG design (not play, design) as a technology; because our techniques can in fact get better. We can recognize where there are flawed behaviors, like the whole "force the rogue to roll Sneak and Move Silently every five seconds, and they are immediately seen and captured/attacked when they fail" problem, which is addressed by Let It Ride, a technique that even many very, very experienced and otherwise high-quality GMs simply do not learn on their own.
We can--and should--identify places where problems commonly occur, or where there is a serious risk of very severe problems, and look for ways to prepare ourselves and those who will come after us against such pitfalls. That's not in any way a deprecation of the importance of human judgment. It's not in any way replacing humans with robots. It is simply recognizing that it is often easy to run into problems you don't realize are a problem, and even when you DO know there's a problem, knowing you have a problem and knowing how to solve it are VERY different things. Giving others--both contemporary and in posterity--tools to address the problems we know we have means they can be focused on dealing with whatever new problems will inevitably arise in the spaces between the up-front problems we dealt with.
The problem (sorry!) there is defining what is in fact a universal problem, because what some see as a problem others might see as a feature or benefit.
It depends on whether the party HAS to get through the door or not, doesn't it? One of the reasons behind "fail forward" is specifically to teach GMs that there is a different way to deal with a "single point of failure" problem. That is, sometimes you're going to only realize something was a single point of failure too late to directly address it--or you're improvising and didn't think that far ahead, or you truly want this to be a single point of failure because that creates tension, or whatever else. But a single point of failure where the only result of failure is "the game grinds to a halt because nothing happens nor can happen" is pretty blatantly a bad thing.
I disagree. I can't think of a situation in which "nothing can ever happen", but I can think of many a situation where "nothing can ever happen given what the party can bring to bear right here and right now". Again, they might just have to get creative and think outside - sometimes way outside - the box.
Fail forward, as a rule of thumb, means that even if the party gets struck by such a thing, the pace of the experience and the enjoyment of play don't get drained away as the party sits there, waiting for one of their schemes to finally, finally, FINALLY open the stupid friggin' door.
One of the loudest at-table cheer-and-high-five experiences I've ever seen in a game happened many years ago. I was a player, we'd met a door we had to get through, and the only - and I mean ONLY - way to get this thing open was to answer a riddle.
We spent two and a half sessions trying to figure out this damned riddle; in hindsight of course the answer was obvious but at the time we just couldn't see it, and when we finally got it there were cheers all round!