Is there a list of plugins that are allowed on a server thats on the community server list?

I'm pretty sure there's no definitive list or any automatic checking of it. The rule is that you can't have any mod that changes the game from the perspective of the players. Only backend admin tools.

As far as I know, this is policed by humans but your server will eventually get booted off the Community list if you abuse the honour system.

Modifications that change gameplay are prohibited
Custom user interface is prohibited
Admin tools are allowed
Modifications that assist you administrating the server but DO NOT alter the game-play. You can inform players, but cannot prevent them from doing something.

So things like GUIAnnouncements are allowed then? I wasn't sure about that.

No they aren't, I've just said custom UI is prohibited.

What about server settings, do they count? For example, I can disable radiation and decay without using any Oxide mods, is that allowed, or does it have to be a completely vanilla experience?

JimDeadlock

What about server settings, do they count? For example, I can disable radiation and decay without using any Oxide mods, is that allowed, or does it have to be a completely vanilla experience?

Anything that changes the mechanics is not allowed. Anything that makes the game appear to not be "vanilla" is not allowed. Facepunch only allows "admin tools", which are tools used by admin users to administrate the server and would not be seen by normal users or be questioned if it is "vanilla" or not.

I've seen servers on the community list with kit plugins ect, is their a way to report servers?

Uh you can write an email to Facepunch