I've been playing the game for a week now and got a time that i liked enough to submit my run (19:16).

I decided to submit my run, and came across two sub-categories: Unrestricted and Legacy ( Inbounds No SLA category). I made some research and found that it depends on the game version/build and available gliches. The question is which versions and what gliches are different between those two, on what category should i submit ?

And last question (I must have misunderstood somewhere), why has the Unrestricted leaderboard better time than the Legacy leaderboard, and less player playing it ?

The exact differences are listed in the category rules. Legacy prohibits vaultskip (in 00), vertical angle glitches (VAG, in 17), and angle glitch saveglitch (AGSG, in e01), and is therefore slower. If you don't do those strats, probably submit to legacy. Unrestricted has less players mostly because of vaultskip, which is very random in noSLA, making it very frustrating to run, along with including a couple of glitches people felt ran against the idea of the category and were grouped in. People tend to prefer how legacy plays, and it's where the competition is and has been anyway.