Now (like Mark) IANAL, and the material in the link is a News Bulletin
not "Qualified Legal Opinion", but taken at face value the right to be
forgotten relates to data about a person, not things they have posted on
Groups.io or other social media. As such they would have no right to
assume or expect that such material can or will be deleted on request,
and of course while material they have originated might be deleted,
where that material is quoted by others those quotations and their
attribution will remain firmly in place.

As recently mentioned on the GMF subscribers can delete their own posts
unhindered, so I would suggest that anyone wishing to have their posts
deleted should be encouraged to set about doing it themselves. As far as
I can see there is no reason for believing that a group owner or
moderator must trawl through message archives removing posts submitted
by any named individual just because they ask for it to be done.