Can the PR still show all the individual commits when you come back to it? If so, are they still in the repo with some keepalive tag or are they persisted outside of the repo along with the PR-comments/metadata?
Yes, GitHub keeps the separate commits visible in the closed PRs. For what it's worth, I never ever found those useful anyway and worked just with the squashed commits. It's up to the team to make sure the squashed commits don't grow to thousands of lines in size.