Is it clear enough, to the person signing up for the Free Plan, as to whether they have legal responsibility for the content of “their” forum? (I’m thinking of the UK’s Online Safety Act but other jurisdictions will have their own legislation.)
In particular perhaps, age verification, reporting of illegal content (porn, exploitation, grooming)
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As with anything, you need to make sure you’re following the relevant laws for where you live.
Thanks! The relevant bit is, I think, this:
Customer remains responsible for the content published through its forum and for ensuring compliance with applicable laws
One reason I mention this is in the context of people migrating from the likes of Discord (or Facebook) where I think it’s the platform provider who does much of the compliance work.
Is that illegal nowadays? 
It might be if your audience is not age-verified over 18. There’s also CSAM, which some people would think of as porn.