They are encouraging this to be used as a permalink service for icons even in markdown files, but what guarantee this will be around in the long term?

There's no guarantee of anything being around.

Anecdotally, I recently worked on a project that used a Flag CDN - worked fine for a long time, then started sporadically failing.

As such I don't think I'd used anything in production hosted on a none static copy CDN like Cloudflare, sure Cloudflare could do down, but we probably all have bigger issues then than my flags not working.

True! but the mitigation for nothing being guaranteed is to … well avoid free public CDNs :-). Unless they are backed by a big company maybe.

Sprawling these kinds of links in your markdown instead if /assets/icons/mylocalicon.png seems a bit risky. Markdown tends to get used in Github / Local Git which can happily just reference the files stored along with it.

Also maybe hugo/jekyll type sites or internal docs. Same thing.

So the effort posted here is great but I would prefer a zip to a CDN link.

Yeah, I suppose as part of a build process you could find references to the icons generated, download them, store locally and use those - the service itself is brilliant, super useful being able to generate those icons on the fly - it's just having it as a dependency for something so key is hard.