I've written a few times regarding not being able to challenge my groups. (Most recently in this thread:
D&D 5E - A Mess of OP Characters (magic items, rest mechanics, etc.))
I want to work on a house rule document to present before the start of the next campaign to keep this type of experience from happening again.
What am I trying to avoid?
What are you trying to avoid? The feeling that the PCs are overpowered and can do anything. The answer to limiting this feeling I'd advocate is not reducing their chaces of survival by limiting their power and survivability. It is instead giving them challenges they can fail without it ending the game. You do not need to change the rules if you instead change what your challenges are. This is advice woven into the DMG, but that could be expressed a lot better and - in my experience - when a DM embraces it, they tend to see their campaigns really reach new heights of enjoyability.
Why do I favor this approach? Because it doesn't %@# on the PCs. It doesn't make them feel like they're nobodies in the world that are struggling just to get by. It allows them to feel like the heroes - but to still have stakes, wins and losses. In each of the approaches I discuss below, powerful PCs can still be in a position to fail - and if they fail it doesn't destroy the game. Instead, it evolves the storylines in new directions.
Rescue Missions: If the PCs are more powerful than their foes, there is no real risk that the PCs will die. However, that doesn't mean that everyone else they care about isn't at risk. What if the story involves enemies kidnapping an ally of the PCs and the PCs needing to figure out how to invade the enemy hideout, and then rescue the ally without the enemies 1.) escaping with the ally, or 2.) killing the ally (rather than letting the ally escape)? That is a very different challenge - and one in which a Vorpal Sword in the hands of a first level PC becomes fairly irrelevant.
Escape/Timed Scenarios: Being able to kill everything doesn't mean they won't slow you down. If the temple is filling with acid or collapsing, and there are 5 treasures in different rooms: The PCs have to decide what to pursue and what they can't get before it is lost/destroyed. The challenge stops being about whether they survive and instead becomes how efficiently they can get small victories. You can set these up so that the PCs die if they don't escape in time, but you can also set them up so that they just lose access to things as the time expires - which is often the better approach for this goal.
Defenders: Similar to a rescue, but the PCs are on their home turf and the enemies don't care whether the PCs survive - they are going after the things the PCs are defending, whether that is people or treasure. A staple of my low level adventures is when the PCs have to defend a small fortification throughout a night when a raising party wants to get in and take something. The PCs may be facing brute force, stealth, magical surveillance, or a wide variety of other concerns. The enemy may realize the PCs are a real threat - and instead of trying to fight them, they'll do everything not to engage the PCs. That first level PC with a Vorpal Sword has to figure out how to get that sword to his foes here without giving up the defense obligations they have. In many ways, this reverses the normal roles of the PCs - instead of being the murder hobo, they are stopping the murder hobo.
Uncertainty: The PCs are in town. They're at their favorite establishment for beverages when a fight breaks out. Both sides start screaming that the other side has to be stopped. One side claims the other is trying to set off a bomb. The other side claims that the first side is protecting a ritual spellcaster that is summoning a powerful Demon as they speak. If the PCs do nothing, if either is telling the truth, something horrible could happen. While the battle rages the PCs have to figure out who to stop, and how to do it without something bad taking place. They have to solve the puzzle, while in the fight, without using their full capabilities. How useful is a vorpal sword if you don't know who to use it on?
Utilizing these types of encounters, instead of just, "Can you survive THIS battle?" encounters really does reduce, if not eliminate, the feelings that underly the types of concerns brought by the OP. The challenges and battles gain depth, variety and stakes that you can't have in a 'just kill everything' game (because if you have a real risk of PC death all the time, you're just going to kill off PCs a lot - which gets old).