I've uploaded early pre-releases of rio-mbtiles 1.6 to PyPI. See https://github.com/mapbox/rio-mbtiles/blob/main/CHANGES.txt#L4 for detailed changes. In a nutshell, the new features are
* Support for 8-bit input and output
* Automatic alpha channel creation for RGB input when the --rgba option is used
* An option to keep empty tiles with no valid data
I'd love to hear reports about the pre-release.