While this system (13" with function keys) has a removable SSD it is a custom unit.
Getting a replacement or upgrading it will be very difficult and expensive! It took OWC the only 3rd party which makes a compatible SSD unit for the older retina class systems over three years to offer one. Presently Apples SSD is faster than OWC's in the 2015 models (PCIe x2 Vs PCIe x4).
I would strongly recommend you buy the largest SSD you can afford. If you still need more storage I would go with an external SSD or HD drive via USB-C.
If you're asking about the 15" model then forget it! It has soldered SSD chips to the logic board. There is no function key version of the 15" model.
Review this IFIXIT Teardown: MacBook Pro 15" Touch Bar Teardown. Jump down to Steps 8 & 9. To compare things here look at this IFIXIT teardown which is the 13" version: MacBook Pro 13" Touch Bar Teardown. Jump to Step 9 to see how its SSD chips are also soldered to the logic board as well.
Only the 13" model with function keys has a removable SSD. But again it's a custom part so only Apple can supply it big bucks $$$ and likely not available for quite sometime and only as a returnable spare.