I did SS for 7 months, but the last few months my gains slowed. I gained some muscle and fat, but they changed the scale at the gym and I don't know how much weight I ACTUALLY gained because the old one was off by like 10 pounds and they changed it half way through so it had me confused. I probably gained 15 lbs, half of which being muscle. Didn't gain much more after that, even though I continued working out, nothing has been very effective after the initial five months or so.
Yeah, that's expected. SS is a beginner program designed to be done for only 3-6 months. When your progress on your lifts stalls out, it's time to switch to a periodized intermediate program such as Texas Method[1], which will have you doing ramped work sets and taking deload weeks to allow for more recovery time between your max effort sessions.
My stats after 7 months: https://imgur.com/Pmnmkc8
Two years later, they barely improved: https://imgur.com/P7qGI5n