The compensated buttstock is described as:
Reduces recoil, but impacts pivoting mobility.
There are two compensated buttstocks available for sniper rifles in Wildlands. The locations of both are covered here.
The first can be found north of central Caimanes province, at a makeshift hospital just off a road. There’s no resistance here, and the accessory case is upstairs in the red building, very easy to collect.
You’ll find the second one in northern Ocoro province, at a Santa Blanca outpost holding rebel prisoners on a hill. A “Food Drop” side mission is nearby to steal a helicopter. The prison is crawling with dozens of cartel for a fun challenge. The accessory case is inside the armoury on the top level of of the outpost.
This outpost is quite large and depending on which direction you approach it from will depend on how you tackle it. But using sync shots to pick off strategic targets is a good start, and bear in mind you can begin to free the prisoners to give you support. Or you could start from the top and possibly get to the armoury without being seen before you exfil the area.
The compensated buttstock is only available for the SR25 (found in Caimanes), and the Dragunov (which you can pick up in Villa Verde).
This is one of those accessories you’ll have to decide for yourself whether you need or not. It slightly increases accuracy and decreases mobility. As accuracy is pretty important when sniping from a distance, you may want to consider this if you use one of these two guns.