> [...] so I assume even with GCC support removed, you can still build FreeBSD, if you insist, with an external copy of modern GCC.
Indeed- "external toolchain" includes gcc6[0] and gcc9[1] ports that can be used specifically for building the FreeBSD base system. These are mostly easy to use, install the flavor for whatever architecture you're building and specify CROSS_TOOLCHAIN=<arch>-gcc6 when you build. More/better/complete information (and examples!) at [2].
[0] https://www.freshports.org/devel/freebsd-gcc6