@pettter @edafe @pavelzinoviev @smallcircles @obsidian precisely.

#Tardnap as another example:
They do release their source code (only source-available, not open source!) to both fulfill Kerckhoff's principle, allow people to audit said code independently as well as port it to their OS of choice.
https://www.tarsnap.com/open-source.html

Personally I prefer solutions like #Zulip that come with a good #API amd thus allow 3rd party clients like #Barnowl to support easily.