A NAS is just a computer and TrueNAS is just Linux (ok, TrueNAS CORE is Bsd).

You can run zfs on any machine: they recommend loads of RAM for optimal performance, which you don't need at home (or at work, unless your job is running a data center).

You can choose from a number of FOSS NAS-specific operating systems, plus all linux distros (since you post here, I'd assume you either can or aim to administer a home sever?)... why would you go with a proprietary OS?

There are several FOSS operating systems for network equipment too (keyword "NOS"), but as far as I'm aware none that work on small soho/edge switches. OpenWrt runs both my router (mikrotik) and WAPs (tplink), but the two 8-port switches I have at home (also tplink) run their proprietary firmware.