Here's an excellent year-end summary from the WordPress plugin repo review team by Mika Epstein. In 2018, they only approved 52% of all submitted plugins. The single most common reason for this is “people don’t reply to emails.”
Mika notes there was an “abnormally high number of closed plugins” this year, mostly as a result of a cleanup of unused plugins. 🗑️
Just seeing these numbers helps me appreciate the time and effort put in by the plugin review team. It's a lot of work.