Can the Joolz Hub + go as carry-on luggage on an airplane?
Also is there a footmuff that fits it?
No, the folded size of the Joolz Hub does not fit the cabin luggage requirement. It's compact FOR A REVERSIBLE STROLLER, but not that compact as the mini travel ultra-compact buggies. If needing an airplane carry-on buggy from Joolz, you'll need to consider the Joolz Aer.
Here's also our 2021 top ultracompact strollers article to find inspiration in the world of cabin-sized strollers.
Do bear in mind no reversible stroller will actually fit the ultracompact stroller folded size. Instead, you CAN take a larger stroller on board, but they will take it from you during boarding to store, and return it to you when getting off.
Footmuff-wise, most universal cosytoes will fit the Hub. As a smaller reversible stroller, best are those that are not overly fluffy. For example, the Lodger Bunker is a very popular choice that will fit very well and not take up too much space in the seat. The original Joolz footmuff will fit great, of course, too - and with a smaller baby, the fluffy ones (like, e.g., the Elodie Details foot muff) will fit inside as well.