I wonder if making Table::save() return null instead of false would cause too much grief to those upgrading from 3.x. That would allow us to add return typehint and avoid explicit type checks.

I am wary of switching to null here. This is an important and frequently use method.

Yup I was vary of that too, so didn't change the return type for save().

The new exception could be a deprecation warning if we wanted to play it safe.

I think we should keep the new exception. If the callback returns null/void then it's auto converted to false. Returning any other value type is just wrong and throwing an exception is the right thing to do.

I noted down the nullable typehint part for the 5.x topics: https://github.com/cakephp/cakephp/wiki/5.x-Ideas
Until then I guess we can live with the status quo.