This has been a long and popular request from the player base.
Custom/Private servers in this game exist (Partnered Server perks) but RARE doesn't seem to care enough to update them for literally years outside of fixing actual server issue bugs in them.
The foundation has been present for years...
Imma keep beating a dead horse since it needs to be repeated:
Custom public servers (Even a payable fee) will rejuvenate those small sub community's back up again. I also WON'T be surprised if it ever becomes a reality in the last dying breathes of the game in order to revive player count back up a bit in say Season 24 when we get there. Probably a feature tacked on alongside a season? But it begs the question if it's worth their time, when it apparently takes a whole team to introduce a spear and a boar in the game or a new cannon type for a whole season... That's the level we are talking about...
Now imagine a custom server with fully fledged WORKING feature set, would take several seasons of updates to it.
PS: The whole ''Tools not rules" and ''Sandbox'' gimmicks they often use is laughable when there's not actual ''tools'' in the game to play with,. NO. Game mechanics and what's splattered in the world are game features, not ''tools''.
SoT is the only game to claim to be a ''sandbox'' without a custom server option for the masses. I don't regard it as a true sandbox experience if I have to spend hours trying to accomplish a single task that can be done with one click in a feature set., there's no crafting/ world building or game modding. It's as barebones as is and you make your OWN adventure in your head which is the ''sandbox'' that they claim...
tl:dr
Game has built the foundation for potential custom servers (SoT Partners) but has never been pushed for years on end and they seem to be actively ignoring this request for years, as evident.