This is a long line -- would be nice to put it across multiple lines.  (Especially since it's already multiple lines when counted with `RSpec.warn_deprecation`).

I think that `expect{}.not_to` (e.g. no spaces) is not very idiomatic -- the more common way I see this written is `expect { }.not_to` -- what do you think about using that instead?

In error messages/warnings I try to follow the backtick convention (e.g. wrap code snippets and identifiers in backticks).  I think it helps make it easier to read.