FerretDB 2.0 adds support for many missing #MongoDB features required by Mailvelope Key Server. With help from @akshay I was running a patched version at https://keys.puri.sm
Now I can drop the patches by updating #FerretDB to 2.0. Though one feature is still missing so I have to continue using an older git snapshot that does not use this feature.
#FerretDB is a #FreeSoftware #MongoDB compatible document DB built on #PostgreSQL. MongoDB used to be Free Software, but not anymore.