One of my players expressed the idea of having "safe areas" where the party could rest and, if they suffer a Total Party Knockout, are returned to...
Here are two different things: (a) safe areas to rest, and (b) restore points.
(a) Have always been in the game. Unless your campaign setting is an especially dangerous one where you're in danger everywhere, characters always have safe places to spend their downtime, be it your character's home, the nearest village or tavern and so on.
But you probably mean safe areas in the middle of a dangerous place such as a dungeon. I think it has been part of accepted D&D tradition (although how common, I really don't know) to allow a party of PC to rest in the middle of a large dungeon, but finding a "safe" area (and identifying it as such!) is usually up to the players' ability. It's something that the DM and players can agree upon, but not all DM would just give that away, and would at least include the possibility of a random encounter to maintain more verisimilitude (and to make the choice of resting in hostile ground a significant strategic choice). Guaranteeing that some areas of a dungeon are truly safe somewhat challenges your suspension of disbelief: it is something I don't like as a player, thus I wouldn't be happy to be asked for as a DM either.
But it's very situational... maybe your typical game involve loooong dungeon crawls. If your dungeon is the entire "underdark", then you can treat it not so differently from a forest or a battle-plagued region, where relatively safe areas should exist but 100% safety is generally not guaranteed.
(b) is a completely different thing, and IMHO it's very alien to tabletop RPG. YMMV, but for me
character death is a very important element in a RPG. A game where your PC cannot die, or cannot die without player's consent, but still death exists
from the PC's point of view, is not uncommon at all in the world of D&D, and I don't have anything against it (as long as the same rule applies to all players).
But a game where you can in fact die, but then you just restart... that's something I would have issues with. Once again, there better be some explanation, whatever unreal. For example, all characters are in fact immortal for some reasons, maybe they're already ghosts, revenants, or angels/demons. Or maybe they are all in a magical land where anyone who dies just comes back again.
If you use such story concepts, I think you'll do fine. Whatever weird, everyone can agree it's just part of the story, or maybe finding out why/how it happens is in fact the purpose of the adventure.
But if it's not part of the story, and it's only just a hand-waved meta-rules, I think it can easily mess you everyone's suspension of disbelief, make the game feel cheesy and nerdy (like playing with an "infinite lives cheat"), degrading the value of winning battles because if you fail you just reset-load-restart. No thrills...
It might actually be still fun for a very, very "stress-light" one-shot game, but on the second night IMHO it may already be too boring (have you ever played a computer game with infinite lives... twice?), so if it's worth one night only why not rather play a fully lethal one-shot game?
