I can try and find alternative links for each case, unless someone has existing copies.

Should external references live under the etc folder or an external folder? This would remove the need for relying on an external site always being present.

Using Wayback Machine

1. srec_emon52
2. srec_fpc
3. srec_signetics

See also marcows/SRecord-deprecated@2e8540a for some website address fixes.

More than 8 years old, might be outdated partially.

I would suggest Wayback machine, if no successor of that page exists.

The local crc16-ccitt.html file was retrieved from Wayback machine.
Should we do it also this way? Personally, I would remove that file and replace it by the link. No need to add different copyright things into the repository.

Perhaps add a github link checker action. Not on PRs, but cyclic once a month or so.
I don't know which one is good, this one was my first google hit: https://github.com/marketplace/actions/check-links-with-linkcheck

I would agree having some kind of link checker on a periodic basis would be beneficial to make sure links are still valid.

I've started playing with linkcheck, lychee and others. Sadly, unless I'm doing something silly (which is never out of the question) they don't seem to be quite up to the task. A lot of these tools are great for picking dead links unless they genuinely 404. Many won't even explore external links as their main job appears to be chekcing integrity of a static website. I'd be thrilled if we can find a suitable one though.

However, if they land on a "not found" page such as http://www.elsist.net/WebSite/ftp/various/OMF51EPS.pdf (linked from srec_aomf.5), the tools will just pass as the http get worked ok.

Open to suggestions.

Regarding the approach to deadends, I totally agree with the Wayback machine approach if there isn't a clear successor. I fixed a couple of links this way last year and it seems solid.