That was one of my first personal projects over 20 years ago now. Didn't get much clicks on my ads though. And once you look in your logs to see what people are searching for, you'll quickly come to the conclusion you don't want to run that kind of service anymore.
I wrote an article in 2013 about why free proxies are free [1] and got so much feedback from people who had never thought about that.
This was back in the HTTP only days (very shortly before the Snowden leaks) and it was a good way to make money because your proxy could just replace all ads on all sites with your Google ads and you could see all communication. Insane from today's view
In 2015 I did a followup post where I analyzed the existing proxies for any manipulation of the original site and found that 80% of the free proxies did in fact change something [2]
[1] https://blog.haschek.at/2013/05/why-free-proxies-are-free-js...
[2] https://blog.haschek.at/2015-analyzing-443-free-proxies/