I a US citizen who has been employed and insured in foreign countries for almost 20 years but I just recently resigned from my job to travel a bit and visit the US for a few months as a tourist before likely returning back overseas to begin work again. Was trying to buy traveler's insurance and notice some policies say "not PPACA compliant" and now wonder if I am there just visiting but not working for 2-4 months, could I be penalized for not having the correct insurance? Thanks.
You are exempt from the penalty for any month that you can use the Foreign Earned Income Exclusion on (and I presume that if you are a US citizen working abroad you would be familiar with this exclusion as you would need to file US taxes on your worldwide income and you would probably be using this exclusion to not actually need to pay US taxes). You can qualify for the Foreign Earned Income Exclusion based on either the Physical Presence Test or the Bona Fide Residence Test.
You qualify under the Physical Presence Test during a 12-month period if you are outside the US for 330 days during that 12-month period. The 12-month period does not have to match the calendar year, and there can be multiple such (potentially overlapping) 12-month periods. You can choose the 12-month period before you visit the US, including the first month of your visit, and also the 12-month period after you visit the US, including the last month of your visit. However, if you are visiting for "a few months", that might still leave some months uncovered in the middle.
You qualify under the Bona Fide Residence Test if you are a bona fide resident of a foreign country. This might be a better fit for you since you have been living abroad for so long and are just taking a short visit in the US without intending to reside in the US.