Unannotated functions are silently ignored by the CI

[epage]

We've seen this in several PRs that have come up.

[epage]

Maybe --disallow-untyped-defs or one of its siblings can help?

https://mypy.readthedocs.io/en/latest/command_line.html#additional-command-line-flags

[jashnani]

We should research this item in the near term to see how many type annotations are missing. This could help us identify any type changes early on and enable us to address breaking API changes sooner.

Moving it near other API breaking items in the backlog.