Is Comet Cache abandonned? A few things make me think so:
1) In the WordPress dashboard, Comet Cache links to websharks.org/, which seems to have disappeared and been replaced with a LinkedIn redirect. A few years ago, it was a full-fledged website.
2) Comet Cache was last updated on February 2017, which means that there hasn't been any update for 18 months! This is an awfully long time. Even the most perfect plug-ins need regular updates in order to keep up with the latest WordPress practices. (By the way, I had to use the Change Log on the WordPress site, because cometcache.com/changelog is completely empty.)
3) I see testimonies on the WordPress site of people who have "purchased the Pro version and have sent 5 support queries over the last month to cometcache.com/support/, none of which have been answered." (Source of the quotation)