The congested status is reviewed when there is a patch.

Gilgamesh is one of the most popular servers in the game and during Endwalker launched it proved to be the most congested of all the servers on Aether, because world visit queues started earlier on it than everywhere else.

Jenova is not always quite as congested in my experience, although it has that status right now. It does have some popular twitch streamers on it, so it is prone to becoming congested on that basis as well.

Even when servers don't have congested status, you still can't make a character on them if the server is effectively congested in a given moment, although you can do paid transfers to them when they don't have congested status.