I’ve found this in the legendary raspberry I2S thread (started 10 years ago, 43 pages !):
Sun Jun 05, 2022 9:10 am:
" Hi, can anyone please explain why the dtoverlay=i2s-mmap is not working in rpi-5.15?"
HiassofT: It has been removed ages ago as it’s no longer needed.
https://forums.raspberrypi.com/viewtopic.php?t=8496&hilit=tdm+i2s&start=1050#p2008731