Seems like the prices are not updated anymore since the GOG's latest redesign(?). But site isn't dead, there are still new releases added to the database so what's the deal with price history? Something changed in gog's api?

The redesign changed how things work so GOG Database's code would need to be changed. However, Yepoleb has other priorities:
https://www.gog.com/forum/general/gog_database_a_website_that_collects_data_on_gog_games/post404