GDPR grants you control of your data, including the right to have it removed. And if you have tools built in to comply with such requests, then you absolutely can remove the "irrevocable" clause from your TOS.
As I say I am no lawyer but will be getting some more personalised advice on wording since we seem to be growing! I hope you can see there are no ill intentions but I understand that it is good to have these things in black and white. Many thanks. :)
Actually, GDPR apply to personal datas and not published content. If you can "anonymize" the content (meaning: taking out the author name and any pseudo-identifiable information) then GDPR won't be a problem.
However, taking out the author name from a content could be seen as a kind of copyright infringement... so the content produced need to be co-copyrighted by the platform