I’m not sure if the issue lies on Discourse. The software itself is FOSS.
AFAIK I didn’t saw any limitation on the on-prem edition, so I don’t get it why a company like Mozilla doesn’t have the resources to handle that.
And, even if they are using Discourse’s Cloud infrastructure, it’s not common for services to reach the end of payment without sending a ton of warnings.
Yes, I've been using it for a few months now, I love it and it is my daily driver. The ux is very nice. It is also very buggy when a new release comes out though, mostly because I don't think there nightly builds have enough users.