Depends on which Teensy model you're using and what CPU speed (as set in the Tools > CPU Speed menu before uploading). On some boards, also may depend on the pin, as some pins use different timers with slightly different hardware capability.
I believe code was added many years ago to support very low frequencies on Teensy 3.x by reconfiguring the timer clock source. Pretty sure that work has not (yet) been done for analogWriteFrequency() on Teensy 4.x, so the low limit is probably around 20 Hz.