I found a free Paper server running on 1.21.11 and it is being tested to see how well the host holds up.
It keeps things pretty simple with only a few plugins. You get Essentials plus mcMMO and MineGuard.
If you want a chill place to hop in and help test performance this one is open for people to join and mess around while it gets tried out.
FREE Paper Server Stress Test
GG.MINEWIND.NET New Server IPGhouls SMP Hardcore Semi-Anarchy Survival
I found a long term survival multiplayer server that is meant to be really hard and intense. It is hardcore style where dying gets you banned for 12 hours so every fight matters.
It runs on hardmode and natural regeneration is turned off. That makes food and healing super important and it feels more like a real struggle.
The server has a small group of active players right now. It also has a few MMO style things like skills and collection tracking but it is kept very minimal and mostly just quality of life.GG.MINEWIND.NET New Server IP