Some third party severs may be temporarily closed. Mojang isn’t responsible for the third party stuff.
This isn’t a supported feature of the game. It was something old never meant to be added, but never removed.
This is a feature, work it out yourself.
Mojang can’t fix issues of third party servers. It’s not a supported bug for the game.
Actually I tried to enable experimental gameplay in my server, and it is working. I can use a structure block, and I can save and load structures. But I can't use my behavior pack that will add custom items!!! It only loaded my resource pack, but not my behavior pack!!!
https://minecraft.fandom.com/wiki/Totem_of_Undying
It was never supposed to...