Old editions of D&D don't really depend on armour for short-term class balance; a pre-3e Fighter or Cleric can still contribute when unarmoured, the Fighter will still outfight the Thief, though they will lack the staying power of armoured fighting since their hp will ablate a lot faster. It's a big problem with 4e since every class is designed to contribute equally in combat with their normal gear; banning heavy armour creates an imbalance where Fighters and Paladins are much weaker and unable to fulfil their Defender role. 5e seems in-between.
For pre-3e I think it's ok to treat it realistic/simulationist and have towns where armour is banned, where pole arms are banned, etc. You can still have ambushes and an unarmoured high level 1e Fighter will still fend off the summoned demon or band of bandits ok.
For 4e it really doesn't seem fair to require an encounter where the Fighter has no armour, and I've never done this. In my 4e game the PCs have been in situations where they need to go unarmoured, eg the Loudwater Summer Ball, but I have never used this as an excuse to attack the PCs. 4e works best with rare combat IME, not random wererats leaping out of the sewers. Ironically this tailored approach actually 'looks' more realistic than the frequent random combat of older editions.
I think 5e can work with either a toned-down version of the pre-3e simulationist approach, where the PCs can occasionally be attacked while unarmoured but keep it rare, or with a 4e tailored approach where the heavy-armoured PCs won't be attacked while unarmoured unless they really go out of their way to get into trouble.