> I see FreeBSD specific code littering upstream open source projects

Any examples?

> Why for example are Jails not compatible with Linux?

What? Jails originated in FreeBSD over 20 years ago? This question doesn't make sense.

The OP was referring to systemd affecting upstream by requiring systemd for things (ie Gnome). Are you saying there are pieces of software you have always used, but no longer can because they directly rely on jails?

I don't even use FreeBSD, but this was a weird argument.