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Ford says it is, and if someone has a drive train or suspension issue this will be just another "water in the fuel" kind of warranty exclusion.
Plus, these days, tire dimensions are becoming more and more critical. On AWDs, it's pretty crazy -- tires have to be within 1-2/32" in diameter so if replacing a damaged tire or if encountering uneven wear, one has to shave the offender down to match the diameter of the remaining ones. Even on truck and commercial suspensions, manufacturers are getting very precise about what they require, with the implication that any deviation becomes a warranty exclusion. It isn't just Ford either.
I get Montauk's concern with towing in extremely remote areas. But this isn't a new thing by any stretch. Ford Super Duties have been sold with 20" wheels since 2005, and they have NEVER come with 20" spare tires. This isn't a new thing, and I've never heard of anyone with an issue because of the dissimilar spare, either with warranty or a failure.
It's your truck, so you should do what makes you happy. But I think you're better spending your time concerning yourself with stuff that actually needs to be concerned about.
I've never heard of anyone with an issue because of the dissimilar spare, either with warranty or a failure.
If the spare is used as a spare is intended(to get home or to a tire store) then it shouldn't cause any issues at all, including warranty. No different than a donut spare in a car. You use it to get where you can fix the tire, not drive it 50k miles.