What is the status of FMM in gromacs?

GROMACS version: 2021.3
GROMACS modification: No

As the title says, following https://pubs.acs.org/doi/10.1021/acs.jctc.0c00744

But what is the trade off for say 10k atom systems?

There is no FFM in am official version of GROMACS yet. Where the crossover will be is unclear. Probably millions of atoms. And fewer atoms when running at extreme parallelization.