It doesn't look like support for FM TOWNS hardware was ever upstreamed into the official Linux kernel source, but Osamu Kurachi has been maintaining patches against Linux kernel versions up to the present-day Linux 6.7.0:
It took me a bit of hunting to find this page, so here's hoping Cohost's SEO juice makes it easier for the next person.
Just for fun, I built a kernel using the Linux 6.9 (nice) source and Kurachi's FM TOWNS patches, and booted it on my FM TOWNS SN. Something is wrong with the console driver though...