The "Seeded" category is made for using previously discovered seeds, yes. If you were referring to worlds already used, no, they are banned across the board.

If a category has this seeded category, you can use a seed to generate your world and it can be accepted (provided it follows the other rules of the category)

Don't call me late to this.

The biggest problem with allowing Easter egg seeds (like FTW) is that seeds done on FTW are always random, with no way of displaying the seed after the run. This can lead really easily for people to splice runs, pre-generating a custom FTW world and simply using it. We have the "show seed" rule for a reason.

Modding the game isn't really a solution either. Forcing someone to use a custom build of Terraria is a big no no, there are certainly trust issues for those who are not modders (despite how unlikely this is) as well as playing on a version of the game that wasn't an official release.