As most everyone who finds this obscure blog knows I started playing OD&D in 1977 and switched to AD&D 1e in 1979 and have been playing 1e ever since. In August of 2022 my 1e campaign, still running, turns 43 years old.
Online I see a lot of statements that are variations of, 'I am certain that no one ever used [AD&D rule X]', usually met with choruses of approval. So let me tell you the rules I used then and now!
Grappling, Pummeling, and Overbearing: Always used them. As a matter of fact, overbearing was a critical element of the fight in Eagle Valley. The base numbers are super-simple to pre-calculate and if you understand the rules it is as fast as any other combat.
Encumbrance, Rations, Light Source Duration: Not only have I always used these rules, they are critical to the game!
Maximum Level of Spell Knowable, Maximum Spells Knowable per Level, Spell Components, Chance to Learn, etc.: These rules are very important to make magic-users work right and I have always used them.
Armor Type Adjustments, Weapon Speed, Area Needed by Weapon: Always used them as one of my players pointed out. If used it means more weapons than you think are great weapons.
Maintenance Costs, Training Costs, Training Time, and Modifying Training Time: I have always used them, although I did adjust maintenance and training costs for some classes, like monk.
Psionics: Always used them, although the tiny handful of PCs with psionics sometimes wish they didn't have them.
Reaction, Morale, and Loyalty Checks: Like Grappling, etc., the modifiers are easily pre-calculated and with practice the use of these rules is very fast and smooth.
Race and Sex based limits to Attributes, Classes, and Levels: Again, always used them, still use them with some modifications. For example, very recently a half-elven ranger in Seaward went to 7th level because the character had earned 5 times the normal amount of experience to go from 6th to 7th.
Initiative, Combat Sequence, and Segments: By 1982 I began experimenting with initiative rules and went through at least 5 major variations of initiative by 1988 until I settled on one that is very close to the rules in the book with a few exceptions. So I have played by the book, otherwise, and am now virtually by the book. I have always preferred segments and my players will tell you I 'count segments' as a way to manage and speed up combat.
I probably missed a few, so let me know what other rules you have questions about.
And I was never alone! I have many friends from then and now that used/use these rules!