I can also confirm Mattermost has threads, which are likeable but a bit confusing.
Would you be open to sharing them in our feature proposal forum (https://mattermost.uservoice.com/forums/306457-general/sugge...) or join the Folded Reply Threads channel (https://community.mattermost.com/core/channels/folded-reply-...) on our community server?
I can also confirm Mattermost has threads, which are likeable but a bit confusing.