New tag: bad source?

After a previous thread prompted me to look at and retag a bunch of old pics, I find that many have a source that gives a 404 or other error.
We have bad id for pixiv specifically, but what of other dysfunctional links? Presumably I should add source request, but I'm not sure if removing the old one is at all appropriate.
So, bad source?

If you make a tag like that, a lot of people who don't understand what hotlink protection is are going to misuse it. Or do you not count that as misuse?

If the source link is protected, is it even a valid source? I don't know. Do we have a clear definition?

The source is the source even if it's protected, if only because that's the only way Find Artist can work well. Providing a friendly link in a comment where warranted is fine.

Right, but if I don't have an alternate source, and the linked one isn't working, do I take some kind of action?

No, leave the URL alone.

Dead links can still be helpful when it comes to searching for new pics from the dead site via archive.org (also for sourcing artist tags via "find artist" whilst doing so)).

And the suggested tag wouldn't be particularly useful, I guess?

I don't think the tag would be a bad idea, and could conceivably be useful at least in so far that it explains the broken link, but definitely leave the source links there even if they are broken.

The issue with broken hotlinks / region blocking is still a concern though.

out of curiosity, what makes a bad_source tag then? there are some direct urls that still work but their landing pages are already gone. are these eligible for bad_source too?