So, for who is using the “community” edition, what are the options to be compliant with EU regulation?
The theme component linked earlier is not sufficient by any means. Even using third party frames it’s not really a solution as it doesn’t allow the necessary fine-grained choice that should be provided.
This is something that has to be deeply integrated within discourse and, chats about EU minding its own business or not, it’s basically a good tool to allow control of its own data to every person. It’s not something to just dismiss because only a part of the world is using it.
Edit to also add this:
Anything that OneBox provide that actually brings in further data manipulation/analysis should be asking for permission the first time as well. Even a simple popup that prevent the action after a message in which the user can choose to agree or not would suffice. Looking at the Reddit Enhanced Suite for example, when you expand a content that requires further access, the first time you are asked to confirm that you want to give permission for that.
To further clarify the importance of this topic. It’s not much the matter of the owners of a forum being malicious or not. I assume that anyone that host a forum in 2023 is kind of a better person than the average 
but it takes only a single disgruntled user or someone who wants to cause trouble just because, to open a big can of worms for whoever is hosting the website and even reach the point of having fines to pay and the forum to close down.