Conversion of a 2011 vintage Unibody Macbook Pro to Linux
There's nothing in NVRAM or in the EFI firmware that make your 2011 MacBook Pro unbootable should you ever want to return to a previous macOS.

As I explained in another answer, NVRAM holds your preboot environment like boot volume.  The worst thing that will happen is it will look for a bootable file where one no longer exists resulting in the infamous flashing question mark or prohibitory sign.  Reinstallation of your OS will fix the problem.

As far as your EFI goes, it's equivalent to your BIOS, just much more advanced.  There's nothing an OS installation is going to do that irreparably harms your EFI firmware.  

Doing installations on separate SSDs is actually a good idea.  I do that myself as SSD prices are now very low that it's more economical (in time) to just swap out a drive than it it is to format and install.  The Macs (of varying vintages and models) don't complain at all.  NVRAM is never reset or variables modified for any reason.

Now, for applicable models with the T2 Security Chip (the 2011 MBP is not), users have to be aware of Secure Boot.  Basically, it will prevent unsigned operating systems from being installed or booted.  However, that restriction can be turned off.  It's basically the same thing as UFEI Secure Boot found on PCs.