Are there any #mattermost clients apart from "the mattermost client" [1] provided by Mattermost itself? Since Mattermost is apparently Expat, #AGPLv3 and #ApacheLicense2_0 for various parts [2], it would be surprising if there are no clients done independently.

[1] restack.io/docs/mattermost-kno

[2] en.wikipedia.org/wiki/Mattermo