Git is a distributed system and there is a copy of all the code in all repos that the code has been committed or pushed into. References between repositories are minimal text flags that mention where a merge commit came from. There is no real link between them.
I have deleted many forks on github in the past with no ill effects. The pull request is "sent" to the repo owner as a patch file. Just try sticking .patch
onto the end of any pull request or commit URL on github to see the patch file contents in plain text. Here is one that someone else requested against a project I maintain: https://github.com/j4mie/paris/pull/35.patch
Therefore it is self contained and you deleting your fork won't effect the availability of this patch to the upstream repo owner.