The atk and at-spi2-atk modules have been merged into at-spi2-core, and they will be released as part of at-spi2-core and maintained together from now on:
at-spi2-core acquired a dependency on libxml2, from at-spi2-atk.
All three libraries (libatspi, libatk-1.0, libatkbridge-2.0) have the same version number. In the git repo this is still 2.44.0, per at-spi2-core’s latest release, but this will now change in lockstep for all three.
Installed headers should still go in the same place as before.
If you are curious about the roadmap for the accessibility infrastructure, please read the roadmap.md file.