I haven't yet run across a question or answer which includes a link to a now unavailable ~.blend file, but I assume that there are some questions or answers with just such a broken link. In some cases, the now missing ~.blend file is an essential part of the question or answer. Is there a bot that checks blend file links to make sure they are still valid, and flags them for attention if the linked file is missing? And whether there is such a bot or not, what is the best way to handle such a situation when one encounters it? I imagine that in some cases flagging the question (or answer) for deletion might be a desirable course of action; in other cases, I imagine it might be of benefit to the community to construct a replacement blend file.
There is not a bot, but there isn't a need for one.
David has kindly written a query which finds all posts of yours containing links to pasteall or blendexchange via the SE data explorer. Here's a fork which lists all .blend links to pasteall or blendexchange found in all the posts on the site.
When you find dead .blend links in other users' answers, leave a comment if you want the .blend (I personally am re-uploading files on a per-request basis for the time being, given the sheer number of now dead .blend links on my answers..).
If the answer relies heavily on the .blend and there doesn't seem to be much chance that the OP can/will re-upload it to a more permanent host, then flag it for deletion.
This goes for pretty much any dead link too, not just .blends.