I can't seem to edit the message now that I know a bit more. So, here's a reply instead.
Turns out that the basic receipt validation does not use SHA-256.
The message rather seems to come up when my app quits with exit code 173 due to the receipt being invalid (as in when the app was copied from a different Mac, and now the GUID doesn't match).
Whereas older macOS versions would then re-fetch a new receipt (and optionally ask the user to authenticate again), this doesn't apparently happen any more (WhyTF not? Why not keep the old method and only use the new method when the app makes it known to support the new method?!)
Forcing a re-fetch of the receipt by simply deleting and re-installing the app from the MAS doesn't appear to work either as the receipt appears to be cached somewhere (again WTF - why isn't macOS being cautious here and falls back to downloading it when the app keeps returning 173???)
It appears the app now has to explicitly request a new receipt. With SKReceiptRefreshRequest, which is already deprecated, but no alternative is offered (I don't do in-app purchases and also don't use Xcode/Swift but something different that doesn't offer access to these classes).
Searching for the error message turned up other reports where people ran into this problem with old programs that cannot be launched any more just because of this receipt caching that apparently doesn't fix itself. I can't understand why macOS is handling this so badly. Is that just ignorance or thoughtlessness on behalf of the responsible dev team at Apple or is there some good reason why macOS can't give older apps and their long-time users a break here?
Or me, who has to spend hours to keep the app working just to keep up with such changes (another one is that suddenly in Tahoe right-clicks into the menu bar item don't register any more if the mouse is at the top edge of the menu bat - which totally violates basic UI concepts established over 40 years ago). This year isn't going well so far for me.