Fanaelialae
Legend
A one class party isn't impossible. It's just more challenging (both for the DM and players). I believe all of the 2e Complete Handbooks had advice for running mono-classed parties.TBF, yes, it has always allowed an all-one-class party, in that choice of class has always been equally weighted, even back in the day when there were stat preq and race limits, everyone could easily end up all playing one class.
Thing is, if that class wasn't cleric, there'd be new characters rolled up every session until you had one or two.And, while later editions expanded the list of classes that could step into that all-important if not exactly popular 'bandaid' role, it's never entirely made party composition irrelevant.
(even the usual exception, 4e, demanded a mix of roles, and thus classes - though all one Source was easily possible.)
I played in a 2e campaign with no casters that went to a fairly high level (I want to say 9th?). My forum handle is actually my character's surname from that campaign. If we hadn't picked a fight with the Kraken (from Clash of the Titans, so basically an aquatic Tarrasque) it would have gone even farther. It was a close fight, so maybe even if our dice hadn't gone cold mid-fight.
Prior to that we only had one or two character deaths, despite regular and challenging combats. The DM mitigated the larger issues by allowing the heal proficiency to recover a few HP once per combat. There were also herbal healing potions that we could create if we were proficient in herbalism, which we crafted constantly. We were often down HP, but the party also had high damage and strong mitigation, so we managed.