The Firebird
Adventurer
If we decouple gatekeeping from the desire to exclude it loses its value as a term. For one, ascending AC is a barrier to people who find Thaco more intuitive, hence gatekeeping. Roll over and roll under systems can both be barriers for different audiences; gatekeeping. The use of ability scores from 3-18 adds complexity without real benefit in 5e; gatekeeping. Overly complex word choices for key mechanics, like "charisma"? Gatekeeping.There very few people who deliberately desire to exclude. There are a lot more people who who inadvertently create barriers, buy insisting on clinging on to things that don't wash with incoming players. But the joke was not aimed at people who like THAC0, the joke was aimed at gatekeepers (intentional or not).
To me gatekeeping implies intentionality. And sometimes people do raise barriers for no other reason than to exclude. E.g. you aren't a true star wars fan if you don't know what planet Nien Nunb is from. It's good to have a term for this.
Thaco isn't this.