It doesn’t seem documented anywhere except for this one blurb in the -help output.
-strict-formats Reject invalid formats accepted by legacy implementations (Warning: Invalid formats may behave differently from previous OCaml versions, and will become always-rejected in future OCaml versions. You should always use this flag to detect invalid formats so you can fix them.)
I notice because
dune turns it on for compilation by default. What thing’s formats is it referring to? What do these errors look like if you run into one?