Shimano Deore thumbies you can make them work for 8 speed, not sure about Suntour.
AFAIK, There aren’t many retro thumbies designed for 8spd as the fashion was “rapid fire” shifters back then
Yeah on one of my pairs you can loosen the spring tension and twist the top collar, which switches the shifter to 8sp.
But, it's possible earlier pairs may have been 7sp specific.