Here's why the restriction on being able to edit closed PRs is a problem:

I was asked to re-submit this PR to a different repo, since I initially targeted the wrong one.

Because the original PR had already been closed, I wasn't able to access the Markdown source of my original PR description. That meant that my only recourse was to copy-paste the formatted content, then go through and re-create all of the Markdown by hand.

There is a lot of it. The unformatted source is 2835 characters, the Markdown grows that to 3020. So, editing in nearly 200 markup characters by hand was incredibly tedious, and wouldn't have been necessary if I could've just copy-pasted the Markdown source from the closed PR instead.

Even though closed PRs can't be edited, it would be better to still allow the edit interface to be opened. Perhaps in a read-only mode (literally just disable editing on the textarea and remove/gray out the "Save" button). That would make it clear that changes can't be made anymore, while still allowing the Markdown source to be viewed and/or copied.