I tried doing this as part of #8134 and hit ~400 new errors when checking our code with mypy.

This can be reduced to ~125 errors (many of which are repeated errors about the same code), by disabling mypy's strict optional checking. This check became enabled by default since we last upgraded mypy. I think we probably should consider enabling this check, but I would recommend disabling it while working on this issue and creating a separate issue to track maybe enabling it and fixing those errors.