That may be a relevant factor but I think a much bigger and harder to avoid problem is that the method table itself is a global mutable state. Hence any function which relies on the state of the method table is itself impure.
That means no generic, multiple dispatch functions should be called inside a @pure function body, which is almost impossible in Julia.
This mutability in the method table may not effect you if you’re not hot-modifying it, however, it seems like it’s liable to cause weird, hard to reproduce bugs if other people were to try and build tools on top of Grassmann.jl, adding their own methods and overloading your functions (or if someone pirated the methods you use, or if you pirate someone else’s methods).
This seems especially worrisome since you do have a tenancy to advertise Grassmann.jl as something people should adapt their project to make use of, so if someone did act on that it could result in an unfortunate situation.