For those that had wanted the One-Winged Angel's signature blade, they've had to rely on a port of an Oldrim mod uploaded in March of last year. Specifically, I'm talking about this right here.

There was a pretty glaring issue in that mod, though: the sword was just way too big. To quote from the uploader in the pinned comment: "Yes, canonically, Sephiroth is roughly 6'1" and his sword comes just below that. Unfortunately, I have no idea yet on how to adjust the size."

So I thought, how hard could it be to resize it? Lo and behold, here's a version that has been tweaked to better represent the actual size the Masamune appears in various media. Yes, it still lacks the curve in the blade that it's supposed to, but that's because I - just like CRAsriel - am working off of what Phate3 has managed to create.

The good news is that you don't need the mod uploaded by CRAsriel in order to use this one, because this one contains everything needed, including the .esp to put the sword in your game.

As to how you get your hands on it, I'll just copy what CRAsriel wrote:
The sword can be purchased from Eorlund, or it can be crafted at the Skyforge (in the daedric category). It can be improved at grindstones and enchanted, and the ingredients are steel ingots, dragon scales, daedric greatsword (or daedric sword for one-handed version), and diamonds.

Credit goes to Phate3 for creating the Oldrim mod, and to CRAsriel for making an SSE port.

CHANGELOG
1.0 - initial release
1.1 - ESL flagging, many thanks to ff7legend
1.2 - Tweaked reach to reflect actual blade size, buffed swing speed and stagger values