Ah, right. I was checking the 64-bit kernel only which doesn't have it as
it's a feature specific to G3 and G4 CPUs:
root@kapitsa:/boot# uname -a
Linux kapitsa 5.5.0-2-powerpc64 #1 SMP Debian 5.5.17-1 (2020-04-15) ppc64 GNU/Linux
root@kapitsa:/boot# grep CONFIG_TAU config-*
root@kapitsa:/boot#