The following chart shows the character level when certain game elements (encumbrance, overland travel, light sources, keys, death, food, nonmagic equipment, camping) can be ignored by most D&D parties. I've charted 1e AD&D, 3.5, and 4e, because I don't have 2e, and because 2e is usually pretty similar to 1e.
Note on encumbrance: I don't know when 1e parties typically get Bags of Holding, but it's probably before level 9. 3e parties, on the other hand, may not all buy Handy Haversacks at level 3, although they can afford to.
Note on overland travel: Would you trust the unreliable 1e Teleport for your overland travel, or wait for Teleport Without Error? How about the more forgiving 3e Teleport? I decided that 3e Teleport's mild risks were more acceptable than those of 1e Teleport, which always carried at least 1% chance of instant death.
Note on light sources: Torches and Light spells are always available at level 1: the level given is the level at which no one in the party need hold a torch or lantern.
Note on keys: 1e Knock can open locks at level 3; the level given for each edition, though, is the level at which the party can deal with an arbitrarily large number of locks in one day.
Finally: Yes, I know that the real level that everything becomes irrelevant is "when the DM says it does." If your DM doesn't track encumbrance, it's never relevant. If your DM wants your 20th level characters to be too poor to buy plate armor, that can happen. This chart is intended to mark the level at which the game designers think a subsystem is no longer the focus of the game.
Let me know if I missed any reliable ways to ignore a subsystem earlier than the level I specified: and help me fill in 2e information.