I saw this question was originally answered on Reddit, but obviously that's now private + I would like to minimise my engagement with Reddit if possible.
Basically: how does the Mythic World First work? Is it based on amount of successful raids? Is it based on time? Basically, what are the rules, if someone could explain.
Nowadays, it's pretty much purely based on when they were released. The current tier includes only the raid Aberrus, the Shadowed Crucible. The previous tier was Vault of the Incarnates.
A "tier" was pretty much defined by a new "tier set" being released. They removed tier sets for a while, but it seems like we're going back to that being a good definition.
Then we would have to say goodbye to any US guild streaming since they would do prog and create strats only for the EU teams to use them and clear faster. Or maybe guilds would do splits while others prog, let the others kill the last boss then clear the tier super fast with established strats.