Matches are made by MMR, not SR.
“If you do decay, it only affects your current displayed skill rating. This decay does not affect the internal matchmaking rating we use, so we can still place you in fair matches.” – Scott Mercer (3)
“Skill Rating decays but your internal Matchmaking rating (the thing that determines who is matched against who – not SR) does not decay.” – Jeff Kaplan (21)
“We use MMR for matchmaking, not SR” – Overwatch official twitter (25)
“SR isn’t used to make a match.” – William Warnecke (40)
You may be confused about what MMR is. See How Competitive Skill Rating Works (Season 10) for a more complete explanation, especially the summary.
Unfortunately, there are also three posts that indicate or imply that matchmaking is either based on SR or matchmaking is limited to a given SR range (5, 7, 35). Scott Mercer is the author of two of the yes SR posts, as well as one of the no SR posts, so he contradicts himself. The contradiction must be resolved somehow, and I decide in favor of SR being used in no way during matchmaking for the following reasons:
Whenever MMR and SR are discussed in the same post (including by Scott), it is to say that SR is not used in matchmaking.
Master+ players regularly see decayed “diamond” players in their matches. And when someone like Seagull decays down to diamond on stream, he is still placed in grandmaster / top 500 matches. If SR was either used for matching, or the limits on matching were restricted by SR, this should not happen.
SR and MMR are closely linked (22), except for decayed players (who only exist in diamond+) so saying matching is done on one or the other is a distinction without much of a difference. This makes Scott being sloppy with the terms not as surprising and makes the answer to this question less important.
There hasn’t been any clear revoking of the old system on this point so the belief that we should only keep the newest non-contradictory statement (35) is implausible.
Generally the people who are insistent that SR and MMR are both used in matchmaking are using it to infer some broken or rigged system with horrible and implausible outcomes that are contradicted by careful analysis, common sense, and unrelated blue posts.
(3) Overwatch Forums
(5) Overwatch Forums
(7) Overwatch Forums
(14) Overwatch Forums
(21) Overwatch Forums
(22) Overwatch Forums
(25) https://twitter.com/playoverwatch/status/850435344457543680
(35) https://us.battle.net/forums/en/overwatch/topic/20759648155
(40) https://twitter.com/ww/status/867570441182826499