What was Richie's secret in It Chapter 2?

My understanding from reading the book decades ago was that Ritchie is possibly gay. This "dirty little secret" doesn't hold up in modern social norms, and in the most recent movies he was portrayed as having a fear of being perceived as gay or bi. In the original text he clearly idolizes Bill and simultaneously feels concern over those feelings. In the modern movies they are fairly clear about his feelings for Eddie, and he is portrayed as closeted gay or bi.

Since you asked about "It Chapter 2", I'm going to streamline my answer to just what was supported in the film: Richie has a flashback to a summer when he was a kid, he's in an arcade with another young boy and it's fairly obvious that Richie is attracted to him. He gets bullied out of the arcade by Henry Bowers, who uses homophobic slurs. He goes to the covered bridge where he carves R + (we don't see the second initial until later) into the wood.

Immediately following this memory, adult Richie is terrorized in the park by Pennywise, who yells about knowing his "secret". The connection between his young arcade crush, the homophobic slurs, and Pennywise's mention of his "dirty little secret" seems like a deliberate cinematic thread indicating that his secret was being attracted to a boy, or boys in general.