Mattermost is really nice. They added voice calling as a beta feature a while ago and it works pretty good so far.
No. I had Manjoro for some time and then Linux Mint as daily driver. But nowadays I use Linux exclusively on the server side privately and professionally.
The strongswan installation itself doesn’t seem to be malicous.
It looks like these packages were installed via the apt repositories: strongswan-starter
strongswan-libcharon
strongswan-charon
libstrongswan