no answer needed, I'm adding the question as reference
The package/update manager of my TeX-system installed in the last weeks a number of packages called latex-XX-dev, and CTAN too shows a number of such packages: latex-base-dev, latex-doc-dev, latex-graphics-dev, and even some executables latex-bin-dev and latex-bin-dev.win32. I saw people referring to them as latex-dev.
Best Answer
Perhaps one addition to Ulrike's answer (it is included implicitly in link given in her answer, but worth pointing to very directly without the need to follow the link):
Our hope is to attract additional beta-testers
We don’t expect everybody to start using the development formats to participate in testing, but we hope that people with a strong interest in a stable LaTeX environment (especially developers and maintainers of important packages) will use the new facilities and help us to ensure that future public releases of LaTeX do not (as has happened in the past) require some immediate patches because of issues that were not identified by our internal regression test suite or by other testing that we do.
Any issue identified when using the development format should preferably be logged as an issue on GitHub, following the procedure outlined in https://www.latex-project.org/bugs/ including the use of the latexbug package as described.
Our bug reporting process normally states that issues involving third-party software are out of scope as we can’t correct external packages. However, in the particular case of the development format showing an incompatibility with a third-party package, it is fine to open an issue with us (in addition, please, to informing the maintainer of that package) so that we know about the problem and can jointly work on resolving it.