The problem with any online game is identifying when an account is truly a legitimate new account and what is an alternate account playing at a high level? How can it be effectively enforced without making mistakes. Overwatch by its very nature is a very snowball momentum game and I see way too many complaints of players that the opponents they face are smurfing when they really are not.
Blizzard has and still does actively stop account boosting and throwing where they can confirm evidence of it. The Overwatch Team in the past have stated they feel it is not a rule violation to own an alternate account, but they have acknowledged the perception of “smurfing” and how account manipulation such as boosting or throwing negatively impacts the game. Most recently, they are investigating ways on how it can be better controlled, but have not made any commitments yet. I will note they have said in the past that new accounts typically are initially paired with other new accounts until that player can successfully establish a stable hidden matchmaker rating. However, I am now under the impression that this level of matchmaking filtering has greatly been removed since the introduction of role queue.
I would like to remind the rest of the community that if you see anyone throwing in games, report them appropriately in the in-game report tool. Those reports do help in more ways than just identifying that player for throwing. Reports help shape what expectations the game has and the Blizzard Global Insights team are able to use that information to help better shape their account policies for Overwatch and all Blizzard games.