[capisce]

Isn't it the case that the offline resources are considered "caches" and thus might be cleaned up if there's not much storage left?

[Vinnl]

Yes, as far as I know, storage space is still limited, so you wouldn't want to be storing GBs of offline emails. I'm not sure how enforcing works nowadays - it used to be that old offline data would be culled (I think that was for AppCache), but that might work differently with Service Workers.