Actually, the words 'force pushed' in the force push message on the PR (or very nearby to that), are a link to a diff between the pre and post push states of the branch.

You can also construct such a diff yourself by going to something like /compare/<old commit hash>..<new commit hash>, and those hashes can also be tags or branch names, and should even allow relative stuff like HEAD^.

(I haven't checked the URL just now, but I use both often. It's a little annoyingly hidden, but it is there.)