Actually, I'd be even happier if any form of offline storage required explicit user permission anyway.
I think this is an excellent example of such an unobtrusive prompt and is how ALL such features should be implemented. Sites should get almost no permissions by default and certainly not be able to show popup prompts.
When the site tells them to "active X permission" without telling them how to (for their specific browser version), most will leave instead.
When the site gives super detailed, up-to-date instructions on how to activate the feature, a very large percentage of users will still leave instead.
When the feature is so useful that many sites go through all thouse troubles and it's common enough for users to encounter this that they'll follow through, most will do so for every site that tells them to and entices them with "ACTIVATE X TO RECEIVE YOUR $10,000 PRIZE, LUCKY WINNER!!!".
network.trr.mode I'm looking at you.
https://dexie.org/docs/StorageManager describes the StorageManager API which lets you prompt the user to allow your IndexedDB data to be stored more reliably. My first thought after reading this article was wondering if this would allow an exception to the 7 day rule... but then I remembered that Safari is the only "modern" browser which does not support the StorageManager API
lol, sucks for users of my client side JS video game!
Even offline storage that is only used locally? Say a game with savegames that has doesn't use online connection to play it.
Another example: a password manager.
But as soon as you allow it any access to network resources then carrying state becomes a liability.