This thread may answer your questions:
Post #5 includes this link:
http://www.nhi.gov.tw/English/webdata/webdata.aspx?menu=11&menu_id=593&WD_ID=593&webdata_id=3143
And at the bottom you can find this text:
If you reside abroad for more than two years without returning, your household registration will be automatically terminated, and you will no longer be eligible to participate in the National Health Insurance program. You will also be required to complete the necessary procedures to withdraw from the program. When you re-establish residency in Taiwan at a later date, you can apply for enrollment again.