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Best FOSS Local Messenger Service?
(beehaw.org)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
This community's icon was made by Aaron Schneider, under the CC-BY-NC-SA 4.0 license.
Bottom line, it can work, if matrix is what op wants to go with.
As for SLL, without it any browser based thing is going to get you a warning, from the browser itself, at least.