Just had this same issue. Research suggests deterministic has been the default for a while, but the F# compiler only added support for deterministic builds recently. Ref:
https://github.com/dotnet/fsharp/issues/7363
I've also got the same issue with using sourcelink which requires portable PDBs. This results in:
FSC (0, 0): --sourcelink switch only supported when emitting a Portable PDB (--debug:portable or --debug:embedded)
For some reason this works fine for .csproj files/CSC.