In the recent update shifting on slime negates all fall damage. I'm assuming runs pre 1.21.3 with slime glitch are still accepted?

The maps are not updated to 1.21.3 yet, so pre-1.21.3 runs are still being accepted.

isn't it impossible to do slime boosts in 1.20.1?

its still possible

is it not allowed to do 1.19 runs anymore because i use 1.19

you can but you cannot do anything that cannot be done on the newest version