There is no SBMM, I have tested it extensively. The longer you stay in a game the harder your next game will be. After a win it takes 5 "bad games" to get back to lobbies appropriate to your skill level.
This is Respawn's attempt at maximising player retention with an algorithm designed to limit your success. In my experience this algorithm has the opposite effect. Frustrated, burnt out players are the ones who stop playing, not players who regularly have fun fair games.
Smurfs would be far less common if Respawn put actual SBMM in the game instead of throwing Plat capped players into pub lobbies full of Master and Pred sweats 5/6 games.