The warnings are bare when using rasterio (neither of those two prefixes).
Packages were all installed via conda/conda-forge on Windows 10:
gdal version 3.0.4
rasterio version 1.1.3
proj version 6.3.1
Also my original post had a typo, I had tried rasterio.Env(CPL_LOG=os.devnull) (not "CPL_DEBUG"). Still a shot in the dark, perhaps.