https://airspace4all.org/not-in-alt-mode-not-transmitting-ads-b-altitud…

If you have a Mode S ES transponder with connected GPS correctly configured to broadcast ADS-B Out you may not be aware that if you deselect ALT mode then the transponder will NOT include your altitude in the ADS-B data stream.

That means any other pilots equipped with ADS-B In Electronic Conspicuity devices will NOT get traffic alerts if your flight path conflicts with theirs.