We've been on Mercurial 5.7 for a long time, and it seems that 5.9 is now close to be suitable.
There is an installation problem to be solved to install it with the Rust modules using pip, as we do.
Basically we're waiting on https://phab.mercurial-scm.org/D11393 or another solution to land. It also looks like we'll need to change the
pip invocation in the install documentation and Omnibus recipes.