Network usage whitelist for cell tethering on OSX?

Solution 1:

TripMode was made specifically for this purpose, and even turns on automatically when connected to designated cell hotspots.

Solution 2:

It looks like Little Snitch allows you to configure whitelists, blacklists, and even more advanced configurations for network access. You can make a cellular profile and switch to it when you use your tethering. It also measures how much bandwidth you've used, per app and total. You can even automatically switch profiles when you connect to your tethered Wi-Fi or Bluetooth network.

Solution 3:

LuLu is the best choice, nowadays.

• It's given for free and it doesn't force you to pay for it (but it's worth spending a dollar or two on Patreon, as gratitude).
• It's open-source.
• Has a simple GUI.
• Allows to blocks the outgoing connections (for the incoming is better to use Apple firewall or something else)
• Allows to choose the apps that should be allowed or blocked

It's really great.

IMHO the only feature missing is that it lacks a passive mode that "blocks all by default". That would be especially useful when tethering or using a limited-traffic hotspot; but it just 5 minutes to block everything (except few system daemons) and then allow just the apps that should have internet access.