1.3b1 is Rasterio's first pre-release containing a nearly complete view of new features for 1.3.0. We're not accepting new features for 1.3.0 and are only working on major runtime and installation bugs. If you would be willing to try out this pre-release and provide feedback on GitHub, or in responses to this email, the 1.3.0 release will be better and will arrive sooner.
There's one new feature since 1.3a4: support for writing both band data and band mask when passing a Numpy MaskedArray to a dataset's write method. Perhaps more interesting is that the binary wheels on PyPI include GDAL 3.5.0rc4 and PROJ 9.0.0, the latest versions of each.
Our set of binary wheels is small right now, but we'll keep working on that, and will send updates when we've added support for more platforms.
Big thanks to everyone who contributed since 1.3a4, particularly Alan Snow, who cleaned up a lot of obsolete code that was supporting older versions of GDAL.