FYI, I just installed QGIS 3.18.1 and added rasterio.  Here's the relevant snippet of the installer output:

gdal (3.1.4-3)
The GDAL/OGR library and commandline tools
Required by: grass, python3-rasterio, gpsbabel, python3-gdal, python3-fiona, qgis-common

gdal111dll (1.11.3-1)
The GDAL/OGR 1.11 DLL (backward compability package)
Required by: liblas

gdal204dll (2.4.3-1)
The GDAL/OGR 2.4 DLL (backward compability package)
Required by: saga-ltr

...

liblas (1.8.0-1)
The libLAS commandline utilities
Required by: grass

So it appears that QGIS actually installed 3 versions of GDAL based on the requirements we have (including GRASS and SAGA).  Do you think this could be an issue?  So far we haven't seen any actual problems because of this.

Thanks,

Ari