How much of 32-bit support is dropped in macOS Catalina?
netkas has experimented with this here.  It seems like Apple has not removed 32-bit capability from the Darwin kernel itself, but merely disabled it as well as removing all the 32-bit libraries from the system.

To re-enable 32-bit processing from the kernel, just disable Apple's disabling function by adding no32exec=0 to your boot-args (sudo nvram boot-args="no32exec=0" if SIP is disabled, if not run the same command from Recovery mode).

To get the 32-bit libraries back, just copy them from Mojave

Sources: Bringing back 32-bit apps to life