Nothing on there says it needs a legacy version, but I may be overlooking something.
It took several attempts (with failures) to get it installed on the latest Raspian version, then after some digging I saw that the requirements said to use "An SD Card with the 64-bit version of RaspiOS installed (please use Bullseye)".
With Bullseye installed, BirdNetPi works just fine, but it is old and comes with old software.