Focusing on computers that can be upgraded to use OS X Internet Recovery:

  • is there any Mac on which firmware can not be downgraded?

I see that for at least one type of MacBook Pro, firmware can be downgraded √

Example

For the MacBookPro7,1:

Background

In some edge cases, the firmware that supports OS X Recovery prevents reinstallation of the the OS that was originally provided with the Mac. For a Mac that is to be sold, this is undesirable.

What to do before selling or giving away your Mac

There may be other edge cases where a downgrade of firmware is either required or desirable.

Related

Internet Archive Wayback Machine views of EFI and SMC firmware updates for Intel-based Macs (Apple article HT1237)

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