GDPR likely doesn’t apply to public facing forums in the way you’re thinking, if you post actual personal data (which has a strict definition) yes it’s murkier, but in general just posting on a public facing forum is extremely unlikely to qualify under right to be forgotten under GDPR.

Notably, GDPR is extremely unclear about this specific circumstance, and will likely fall to practicality. The user can make requests for their data to be deleted, those should in general be followed no matter who’s server it’s on, but they have to be given to each server by the user. Following the deletion requests is generally advisable, but again, it’s highly unlikely GDPR applies here. Feel free to get a GDPR lawyer to actually weigh in though.