I have a question about this.

Per google, shortened links “won't work after August 25 and we recommend transitioning to another URL shortener if you haven’t already.”

Am I missing something, or doesn’t this basically obviate the entire gesture of keeping some links active? If your shortened link is embedded in a document somewhere and can’t be updated, google is about to break it, no?

About to break it if it didn't seem 'actively used' in late 2024, yes. But if your document was being frequently read and the link actively clicked, it'll (now) keep working.

But as I said in sibling comment to yours, I don't see the point of the distinction, why not just continue them all, surely the mostly unused ones are even cheaper to serve.