I literally work in this field and can tell you that "likes" are regulated under GDPR just like any other personal data.
If someone asks us to delete under GDPR, we have to delete these as well.
If someone asks us to delete under GDPR, we have to delete these as well.
Being roughly familiar with GDPR, I would be reasonably certain that you could keep a like as long as the identity it's attached to is deleted. Much in the same way as you may see reddit posts that just show a [deleted] user.