Meet Bob; Bob left some comments on a post on stack exchange and some over zealous moderators decided that they were not needed and so to delete them. Bob wishes to access these comments for the purposes of his own reference. Are these comments a) remaining on SE’s servers, and b) available to Bob under his GDPR right of access?
The answers are "yes" and "yes".
The first "yes" being based on my experience as a moderator: if I can still see the "deleted" comments, then obviously they still exist in SE's data storage.
The second "yes" is based on "Art. 15 GDPR - Right of access by the data subject".
While I have no background on SE's company structure and where each part is located and which juristiction applies, as far as I know SE complies with the GDPR, either because they have to, or because it's easier to, or because it is the right thing to do.
Please note that you have no right to selective access. Companies are well within their right to make the economic decision to automate this and give you a data dump of everything they have on you. It would be up to you to find this specific comment in the data dump of "everything".
They are also not required to do this free of charge, if you ask for this repeatedly.