[slowstart]

Connection migration is part of the current Internet-Drafts. The generalized support for multi-path (i.e. usage of more than one path at the same time) is postponed to a future version of the protocol. You can follow the standards work here: https://quicwg.org/

[modeless]

Good to hear, but it seems like using both paths at the same time would be necessary for connection migration to work well, as you're often not sure which connection is actually better. If you have to wait until you're completely sure one connection is gone before switching wholesale to the other, that removes a lot of the benefit of connection migration.