The religion invokes ‘Migration’ (‘Hijrat‘) as a compulsory requirement (unless it’s impossible under the circumstances because of barriers in movement) when Muslims are forced by law or other means to not follow their religious directives. Here, religious directives mean the ones which are obligatory on Muslims to follow. The distinction betweem obligatory and recommended directives is necessary to be drawn here, becuase if law of the land is prohibiting something which is only a recommended act or an allowed act in religion, then it doesn’t become a religious requirement to migrate from that place. A Muslim can still be a good Muslim while obeying those laws of the land. The example of polygamy is just an allowed act, it’s neither a compulsory directive nor a recommended act, therefore a Muslim will still be following religion when law of the land imposes any such restriction on polygamy.

Following article by Dr. Khalid Zaheer Sahab sheds light on this topic w.r.t concrete areas of conflict:

Humans Rights Law and Islamic Shariah: Areas of Compatibility and Conflict

You may also want to refer to the following responses by Ghamidi Sahab addressing some application matters w.r.t apparent conflicts between Islamic laws and personal laws in the West:

Discussion 45875 • Reply 46055

Discussion 45875 • Reply 46054

Please also refer to the video below from 28:36 to 36:05

Alternative Link: ilm-o-Hikmat 01-08-2021