The clients are mostly Apache 2 licensed, and at least Riot Web has config options to swap out some of the branding at deploy time: https://github.com/vector-im/riot-web/blob/develop/docs/conf...

It's not a marketed use case though, so I'm sure you could find places the app will still refer to itself as Riot (or now Element).

Alternatively the protocol is open so many third party clients exist: https://matrix.org/clients/ . Feature support in third party clients is pretty unevenly distributed though, E2EE in particular is supported in Riot and Seaglass, experimental in weechat-matrix and nheko, and absent in basically every other client.

Got it..

EMS lets you point your own DNS at your client as well as customise the branding. Beyond that, you can run/fork your own client too.