someone asked me to recommend some STG currently on sale on itch and I gotta say, the pickings are slim—putting aside the fact that so many games on itch are free to begin with, a lot of devs/pubs with games on multiple storefronts simply don't bother putting their itch versions on sale, or coordinating with big promos or whatever, so a lot of the games one might hope are discounted are still at full price
tl;dr: Demonizer, ZeroRanger, plus Ex-Zodiac or Bot Vice if you're not too prescriptive about genre