Just a quick question - if someone asks for their account to be deleted how does that affect their posts
Not entirely sure how GDPR comes into play here
Does not affect their post, they will remain.
Well... it depends how the account is deleted: you can decide to remove them or not (usually).
If left, the posts will keep the nick on the poster.
The GDPR comes into play by the fact an IP address is also associated to the posts. But, as long as you define in your privacy policy how you intend to deal with this information (e.g. it will be kept as long as the post remains in place or for at least... a certain amount of years, let's say 10 for example), then there should not be much problems.