If he wanted to give it to the public domain, you don't need creative commons. CC0 is public domain, but you don't need to use it, the copyright holder just needs to state "I dedicate this media to the public domain" and bam.
And any copyright they still have over their media isn't "forever", it will expire eventually. In the meantime, yes their estate likely has the rights, whoever that may be.
There are jurisdictions in which public domain doesn't really exist (e.g. Germany) where you need something like CC0.
I vaguely recall Richard Stallman bad-mouthing public domain in favor of free licensing. I don’t recall the details but I imagine the artist would at least want attribution, which I doubt the public domain ensures.
Of course if we could have predicted his death there are many trivial ways we could have ensured his will is expressed and executed. But it may not be a lost cause.. whoever controls the estate would likely know or believe that he wanted to liberate his work.
Nawor3565’s comment is spot on.
About the link, though: You could link to Archive.org’s cached page of the license if you don’t want to directly link to the CC cloud flair page.
That doesn’t really work. The URL is an inherent part of the CC license. Archive.org might do link replacement but that does not change the license.. it merely corrupts/misrepresents the license. Users of CC-licensed work are still bound by the actual text of the license. IIRC the URL was to refer to something that changes over time, thus making the license dynamic when archive.org works off of snapshots.
Since the second half of a Wayback link is the original URL, a savvy user could easily visit that link when they want to.
indeed that’s the other problem with using archive.org.. the CF dependency is not only still in play but some tech savvy users would still be licking Cloudflare’s boots.
It depends on which license you look at. E.g. from https://creativecommons.org/licenses/by-sa/4.0/legalcode.txt:
c. BY-SA Compatible License means a license listed at creativecommons.org/compatiblelicenses, approved by Creative Commons as essentially the equivalent of this Public License.
It was a while since I looked at this but IIRC the BY license was fine in its early versions but none of the others were. Then I vaguely recall seeing links in later versions of the BY license as well (but that may have been hasty reading on my part since i'm not seeing it in the BY 4.0 ATM). Anyway, it’s a mess.