Bug .NET dlls. Loading assembly failed: File does not contain a valid CIL image

after upgrading from 2020.1.17f1, to unity 2020.20f1 (or any version past that), i get this error on my managed dll plugin:

whereas it worked perfectly in the version prior. preverify says everything is fine.
this is almost certainly because the .net dll was not made in C#.
did unity finally decide to crack down on other .net languages, or was this an unintentional depreciation?

 

what a crazy language, shame its gone