What's a good replacement for MenuMeters on the newest macOS? https://www.ragingmenace.com/software/menumeters/ says it does not support the newest version of macOS any more. 

What's a good replacement?
The best replacement for MenuMeters is: MenuMeters!

It's a port to OS X El Capitan 10.11 and later of our beloved utility MenuMeters by Alex Harper, at http://ragingmenace.com. As he has not yet released the port to OS X 10.11 El Capitan, I made a minimal modification to the source code he has kindly made public so that it runs fine.
Confirmed to run OK on El Capitan 10.11, Sierra 10.12 and High Sierra 10.13.

Take note that currently the above port is somewhat lacking in releasing a new build. To get all the latest updates and features you have to build the binary yourself.
Or: go to another fork at emcrisostomo/MenuMeters which includes the latest fixes and has an installer DMG.
Or: go to yet another fork at axet/MenuMeters which adds new features on top.