I'd forgotten Hotline (I was only 8 years old in 1998 and never used it) but the logo looked familiar.

GLoarbLine PhareRouge seems to run on Mac OS 10.13.6, but I don't know any IP addresses to connect to.

https://hotline.fandom.com/wiki/Clients

The closest spiritual successor, I suppose, would be SoulSeek.

Hotline is still around. There's also KDX. You can get clients here[0]. Hit the default tracker (hltracker.com) to find servers :)

[0] https://preterhuman.net/gethotlinekdx.php

Many times i've dabbled with recreating it, but with a more modern approach. Is anybody out there also interested in this?

Don’t forget Carracho, a Mac specific Hotline replacement.