opam repositories currently have a "one file per packages/versions" but as the number of packages grow it creates a sustainability problem for people with low number of inodes for their filesystems (e.g. see #5484) and a performance problem (you have to open each file on every opam update)

I'm not set on a particular format for that file but it could be the format that opam switch export already uses.
@mseri also suggested using SQLite