I got it running on macOS 10.14.6 (Mojave) and it probably works for higher versions of macOS as well (?).
[7 Days to Die Alpha 20.5 (b2) and Undead Legacy 2.5.82]
Starting Undead Legacy
1. Open the Terminal application.
2. Copy & paste this and hit enter: cd Library/Application\ Support/Steam/SteamApps/common/7\ Days\ To\ Die
3. Copy & paste this and hit enter: ./run_bepinex.sh
3.1 It may take a few moments till the game starts.
3.2 Don't close the Terminal window as long as the game runs.
3.3 macOS might deny to run the command due to security reasons. (Thanks to
Spladow for
mentioning this!)
3.3.1 Go to "System Preferences > Security and Privacy" and click on the "General" tab.
3.3.2 Near the bottom, you should see an option where it lets you override the security concern for bepines.sh.
3.3.3 Return to Terminal and run bepinex.sh. This time it should launch successfully.
4. Have fun.