I'm not saying anyone is making a judgement call and honestly I don't understand why this is a touchy subject. I've just never really gotten clarification on how people run this, or even why they care.
So for me, if an action can't succeed if they roll the dice it doesn't matter if the result is 30 or more. There is no 5% chance to climb the wall because it's a perfectly smooth or magic wall and they need to find a different alternative. So I don't negotiate either: if I've already determined that there is no chance of success or failure the attempt is still made.
But I don't see any real difference if someone rolls and says "I do an athletics check of 30 to climb the wall" vs "I try to climb the wall". Conversely if they roll an athletics check and get a 0, I may let them know they climb the wall anyway and that no check was necessary. Hopefully I've described the scene well enough that won't happen.
To put it a slightly different way: from the PC's perspective they are trying to climb a wall. The way we express that attempted action via the rules is an athletics check. Whether or not it was even possible to climb the wall may be unknown to the PC at the time they make the attempt (especially when magic/illusion may come into play).
So let's say I've set up the scene where the party has been temporarily split up, and each come to a different section of a wall of an ancient keep. I'd be okay with any of the following.
PC 1: Rolls the dice "I make an athletics check 20 to climb"
PC 2: "Looking around, do I see anything unusual about the wall, or any way up that I may have missed at first glance?"
PC 3: "This keep is well known, can I make a history check to see if I remember anything?"
PC 4: "Hmm, my athletics isn't great, maybe this rock soft enough to make handholds for climbing." picks up dice "Do I get advantage on an investigation check because of my background?"
And so on. Each PC is doing their best to get over the wall using what they know they're good at. How would you run it differently? Why would it matter if the skill check they're trying isn't necessary?
Or throw in an additional twist. The wall looks smooth, but is really pock-marked with holes that's covered by a glamour. Anybody could climb it if they wish but the PCs don't realize that.
Again, I'm curious how other people run this. I don't think one way or another is "right", "wrong" or "bad" but I'm always looking at different ways of doing things to see if it's something I can learn from.
Here’s how it’s different if I’m your DM. Same setup with the glamoured Wall.
PC1: I’m going to climb it.
DM: ok, how? Free climb? Use your climber’s kit? It looks pretty smooth to you.
PC1: ok I’ll use my climber’s kit and sink a piton in first as a base.
DM: great, using your climber’s kit you ascend the wall. As it turns out, it’s not nearly as smooth as it seems to be and you move up rather quickly. Unusually quickly, considering the appearance.
PC1: It’s an easy climb, guys.
PC2: Hang on. Unusually easy, he said. I want to find out more.
DM: ok what do you do to find out more?
PC2: Can I roll arcana?
DM: Well, what are you doing so that arcana applies? Describe how that comes into play.
PC2: Gotcha. Ok I’m looking for signs of spell craft, feeling for traces of magic, that sort of thing.
DM: Sounds good. Go ahead and roll intelligence (arcana) or (investigation) since you’re actually feeling. If you get a 15, you’ll know what’s up exactly. If you get a 10, you’ll have an inkling.
PC2: I’ll use investigation then. (Rolls). I got a 17.
DM: OK, feeling the Wall, your fingers find numerous purchases, like handholds. It seems someone has put some sort of illusion to hide the handholds. Why, you cannot guess.
PC3: someone enchanted this for a reason. I wonder if there are traps.
PC1: I just climbed it fine!
PC3: Yeah, but you’re an expert climber. I’m going to check for traps. Magical and mechanical.
DM: How, exactly?
PC3: you said it’s got an illusion, so I know just looking at it won’t work. I’ll have to poke around with my tools to see if there are any triggers.
DM: Alright make an Intelligence check and you can apply either investigation or your thieves kit bonus. If you get a 20 and it’s trapped, you’ll find the trigger and be able to bypass it. If you get a 15, you’ll find the trigger, only. Less than that and you risk setting off any traps.
PC3: Ok I have double proficiency with the kit, so I roll a 22.
DM: There is indeed a magical trap. Some of the handholds magically close, trapping any climbers.
Or whatever.
But note that anybody who just climbed without stating an approach would’ve been caught by the trap. Since I don’t want to use my players’ ignorance against them, I make sure to always ask how they’re doing something, so that I don’t inadvertently trick them or they don’t unknowing stumble into something. They always have to choose to. We’re not going to backtrack to “I would’ve used my kit!” “But you didn’t say so!” “But I would’ve!” And so on. I’m just going to ask, so we’re all clear.
It’s my job to give your character a fair shake. I want to avoid ambiguities that result in a bum deal. I once had a dm tell me I couldn’t draw my sword because I’d left it in a dungeon 2 weeks ago (when I dropped it to administer healing). Apparently because I didn’t say I picked it up after, my character didn’t notice it was missing for 2 weeks. Now that’s bull crap. That’s not every dm, either. But it sure as hell won’t be me. I’m gonna clarify with you before the roll.