You can teach it (given it's speaking/conversation) without technically getting in trouble...you can't give official tests/exams/grades/assessments that will lead to the IGCSE certificate.

It would really depend on the immigration officer reviewing your case were you to get in their crosshairs:

If you just taught the speaking component of IGCSE without contributing as a "proper full time" ELA teacher (meaning you don't give graded tests/assessments/exams etc that would go on the system), it wouldn't be too different from an EPIK NET.

Whereas if you taught the full IGCSE, giving grades and reports that would contribute to your students' final GCSE certificates, while on an E-2 visa...you would be in trouble

China's system is vastly more straightforward, though you're only really meant to do the type of work your z-visa permits...the PSB knows what happens in schools (z-visa to teach ESL, but actually teaching geography), but turn a blind eye to it. In stricter cities/provinces (e.g. Jiangsu. Guangdong), they do summary visits to schools every once in a while to ensure teachers actually teach the subjects their Z-visa permits.