rEFInd boot manager not showing EFI shell
You need a compatible EFI Shell program (called shell.efi) in your EFI partition.

As mentioned in the rEFInd config file documentation, in the showtools parameter section:


  Note that the shell, memtest, gdisk, apple_recovery, mok_tool, and fwupdate options all require the presence of programs not included with rEFInd.


The rEFInd CD version does include an EFI shell (there is also a link for it on the Installing Additional Components section of the documentation) but this doesn't work on my macs (all 2015 or older) as the Apple EFI firmware is too old.

You can download an older EDK version 1 binary which should work on any version : 


Download Shell_Full.efi from tianocore/edk2
Rename it to shell.efi 
Mount your ESP with mkdir /Volumes/ESP followed by sudo mount -t msdos /dev/disk0s1 /Volumes/ESP
Copy shell.efi to directory /Volumes/ESP/EFI/tools where rEFInd will look by default (make directory if it doesn't exist).
rEFInd will display shell as a small icon underneath the OS icons if it finds the executable.