>Instead what people usually do is to say "everything I do from now on is closed source. You can maintain a fork of the old version, but good luck keeping up with my version". Or alternatively "everything I do from now on is under [GPL/AGPL/similar restrictive license], if you want to use it beyond that contact me for a more permissive license deal".
Yes this is what I was describing as reasonable. "Everything after this is governed by X terms" is reasonable. But the whole thing can sound like even if you change terms, previous licenses would still apply, which would be wrong.
To circle back to the specific case of the VSCode extension URLs, at the time they were "new stuff" they were published in a repository with an MIT license notice, effectively publishing them under MIT. For that version that license applies forever. If they change the URLs and keep the change proprietary that's fine, they just can't take back the past.
Though it should be added that I'm just expressing the common understanding, barely anything surrounding open source licenses was ever actually tested in court. There are also some obvious legal positions that would completely change this: does every change need to state the license, are open source licenses actually legally binding etc. However nobody would ever argue those positions because they are detrimental for everyone (ok, the latter one was once argued in a GPL trial, but the court decided not to decide on that)
Yes this is what I was describing as reasonable. "Everything after this is governed by X terms" is reasonable. But the whole thing can sound like even if you change terms, previous licenses would still apply, which would be wrong.