I don’t know of an official date. There is an announcement for a new client software that supports end-to-end encryption on Nextcloud. For this feature, you will probably need the new client:
It is announced for NC 13 which is end of the year. The ownCloud client probably stays compatible, it’s hard to say when they introduce something that breaks the compatibility completely. I would switch next year when the new NC-client is working without major hiccups.
Well, as soon as client side encryption gets fully operational you won’t be able to access these files anymore with the Owncloud cliend (except they would implement this, which I doubt because of their proprietary solution).