It's more common than you might think. I know of at least one popular email client that stores your credentials on their servers to enable features like multi-account sync and scheduled sending.
It was the Ethernom Beamu, company now defunct.
>I would expect such a feature to use end-to-end encryption for the data
How would "end-to-end encryption" when such features by definition require the server to have access to the credentials to perform the required operations? If by "end to end" you actually mean it's encrypted all the way to the server, that's just "encryption in transit".
This is what Zoom claimed was e2ee for a little while before getting in trouble for it.
https://news.ycombinator.com/item?id=45458482
Its entirely their end to their end encrypted. You don't get any privacy.
If we had competent regulators this sort of blatant willful negligence would constitute false advertising.