Unless the original license includes a revocation or termination clause, it cannot be unilaterally revoked or terminated. The OGL does include a termination clause but it doesn't apply in this context
A license doesn't limit the rights of the licensor in any way that is not explicitly mentioned in the license text. So any license can be implicitly revoked if it doesn't explicitly say it is irrevocable (e.g. the GPL says exactly that, to avoid this kind of loophole).
The GPL v3 uses the word "irrevocable". The GPL v2 and earlier, and many other open source licenses, don't.
But that the authors of GPL v3 wanted to close this gotcha loophole that you and the forum lawyer calls attention to, doesn't mean the gotcha loophole would have worked. Nor that it's going to work, or even will continue working even if it's worked before.
Courts aren't computers just executing legal code - for good and bad.
Sure, nothing is certain related to legal matters. I was looking into this some more and there are apparently even arguments that even the GPLv3 is actually revokeable as long as all the copies were given away for free. Of course, others argue that even the BSD or MIT license are irrevocable.
There is little case law about software re-licensing at least, so yeah, it's hard to say.
Where in US statutory law or case law is there an implicit right to revoke a license unilaterally?
There is plenty of case law that says the opposite. For instance Cohen v. Paramount Picture shows that copyright licenses are not unilaterally revocable even if the original license doesn't explicitly state it is irrevocable.
A license is an enforceable contract. Revocation requires the consent of both parties, if no exception is provided for originally. Eisenberg, The Revocation of Offers covers many of the nuances of when a contract or even an offer is no longer revocable.
I don't speak legalese, but does the absence of a termination clause mean it cannot be terminated, or does the presence of a non-terminable clause mean it cannot be terminated?
That is, if it's not explicitly mentioned, which one applies?
There is a termination clause which lists the acceptable reason as being a failure to fix a breach of the license within 30 days of being notified of that breach.
The fact that there is no "we have a new version" clause listed in the termination section would likely go against wizards but they are likely to claim deauthorization is different to termination.