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Another alternative you could consider is Zulip. Unlike Element/Matrix it does not support E2EE (at least as far as I know) but it is fully open source software with a pretty active community. Also unlike Mattermost they provide free cloud hosting and don't lock certain features behind non-OSS enterprise versions.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.