The default DM will still be NIP17 (supports sync across different devices/clients), while secret chat and private group will be replaced with NIP104 (does not support sync across devices/clients).
Discussion
I see. Noob question here. Is sharing a single device key across multiple clients through an encrypted event on relays a bad idea?
It’s not a bad idea imo, but rather a trade-off. Sharing the device key increases the risk of leakage, which goes against the intention of forward secrecy.