BcMath\Number class is a class for an arbitrary precision number. These objects support overloaded arithmetic and comparison operators.

https://www.php.net/manual/en/class.bcmath-number.php

Would it be possible to support the BcMath\Number objects in a forbidArithmeticOperationOnNonNumber rule?
Arithmetic between BcMath\Number and BcMath\Number or int is supported. BcMath\Number and float give a deprecation warning (https://wiki.php.net/rfc/implicit-float-int-deprecate)

I could try to implement this addition to the rule and provide the PR if there will be any support.