From what I understand it’s about the fit. 

Anything above 2TB will likely be double sided and thicker SSD unit therefore won’t fit nicely in the thin slot available in the 2021 blade 14

That being said there is a vid by the Unlockr on you tube installing an 8TB Sabrent, albeit with a bend in the drive.

Just watched an installation vid of a 4TB WD SN850X into the 2023 version where the guy removed some felt pads that sit between the SSD and the underlying components on the board. He swapped them with a thinner thermal pad to make the 4TB double sided SSD fit. Was wondering if that would be similar in the 2021 but not sure. I understand the 2023 is a bit thicker laptop and they were able to make the RAM up-gradable. The 2021 is quite the compact design so there will be limitations as to what will fit.

Thanks for the reply. I didn’t even consider the physical form factor.