Maxperson
Morkus from Orkus
That's flat out untrue. In AD&D, adventurers were considered VERY rare. One of the bigger changes that came with 3e was the guidelines for leveled NPC's in a community. And those guidelines were heavily criticized as creating very high powered worlds.
But, no, leveled NPC's were certainly not a thing pre-3e. Mostly because anyone that wasn't an actual character class, had monster stat blocks. A Bandit wasn't an X level PC. A bandit had d6 HP and a completely separate stat block from, say, a trader.
No, classed NPC's were a very rare thing in AD&D.
Other than them being in the vast majority of 1e modules and supplements you mean. Without even combing through these modules for more...
DMG 1e: every single encounter table has character(NPC) for the PCs to encounter.
I1: 20 characters are listed to be used as PCs or NPCs.
I2: Has 33 listed in the first freaking meeting with the PCs, though only 4 are higher than level 0. The random encounter table included clerics. I saw a bard, and I didn't go very far into the module.
I3: Has at least one NPC cleric in it.
I4: Has three classed NPCs in it.
I5: Has five classed NPCs in it.
I6: Has ten by page 9 and just continues to climb from there.
I7: Has enough that it included an abbreviation legend for Cleric, Thief, Fighter and Magic User. I didn't bother looking further as it was a given that there would be plenty.
I stopped the I series at that point and looked elsewhere.
B1: 5 classed NPC bodies on page 9. The end lists classed NPCs and says that they are easier to find that henchmen. It then goes on to list the names and stats for 48 classed NPCs.
Yeeaaaaaaah, they're rare alright. 8 for 8 on modules, and listed on all the encounter tables in the DMG. I'm not going to look further as they are all over the place.