Using brew, I got "Apple could not verify `whosthere' is free of malware that may harm your Mac or compromise your privacy." [Move to Trash] [Done]

It just means that the binary is not notarised. You can go into Privacy & Security to override.

Unsigned binaries on macOS have slowly but surely been marginalized more and more with scarier and scarier warnings and harder hoops to jump through. You can enable execution in the system settings “Privacy and Security” pane.

I’m sure this has nothing to do with Apple’s subscription-based (and government ID requiring) developer program membership which is the only way to get such signatures.