Can I book a return ticket for 90 days in Thailand if I apply for a 60-day tourist visa and plan to extend it for 30 days?

This post is not going into the "this airline at this airport checked or not"-poll. Thank you for the information. You need to contact your airline by yourself to check.

I have been denied boarding with a 60 day tourist visa and a return flight past that date. Only once (Jan 2023) has immigration asked to see my departure plans. I go to Thailand annually

Whether you need onward travel within the days you will be stamped in for (in your case 60 because you're getting a tourist visa) is dependent on the embassy/consulate you're applying at

AND

is controlled by the air carrier too, and has nothing to do with thai immigrations at passport control once you get here.

If you get the visa approved showing onward travel in 90 days out check with your air carrier to see if they have a rule about needing proof of onward travel within the number of days you'll be stamped in for.. If they are okay with you boarding the plane with onward travel 90 days out you'll be fine once you land here..

ATTENTION: ALL MEMBERS, the proof of onward travel within the number of days you'll stamp in for (on a tourist visa or free entry) is far more an AIR CARRIER policy than anything to do with thai immigrations. Check with your air carrier on their requirements.