Everyone used house rules. The funny part was how few people seemed to realize that the rules they were using were, in fact, house rules!One thing that amused me at the time, as I went to conventions and pick-up games (1e AD&D), I'd ask the DM "do you use any variants?" and they'd be like "nah, by the book" (or, "yeah, we use a mana system...") ...
... then, like d10 initiative, critical hits, possibly even fumbles....
The Forever DM in my AD&D circle had a rule where a 20 was a crit and a 19 gave you +2 damage. It was a fun conversation years later when I was playing D&D with one of my old friends from those days and he was griping about how "they got rid of the +2 damage on a 19 in 5e".
"My man, that was, and never has been, a rule in any book."
"What? No it was, I used it for years."
(Gets out the books and we go over them)
"Man, I could have sworn...maybe it was in a Dragon magazine..."