I don't recall any service like that. It sounds good, assuming the market still have an interest on free trading calls. I doubt you can find out their private/paid calls since you'll have to spend a lot of money doing that.
Most traders that I know of follows Telegram channel regularly to check signals, some of them also use Discord. I personally only follow The Dude and Coin Bureau, not because they give trading calls, but because they shared lots of good info for free.
The market is full of that and there are hundreds of thousands of followers. Demand is not the issue.
My interest was if there is somethinig already existing or people are just guessing whom to follow or doing their own check ups (it's just not that easy to check all the past signals on your own)