First of all, a question - why do you think, that account history is not in effect?
How would that work - simple, Blizzard has all of your gameplay stats and is able to compare them to in game stats of other people, playing with the same heroes you do (PBSR). Certain ranks have certain performance indicators, otherwise “Bronze to GM” raids would occupy a lifetime instead of days. For example, I’ve seen a Fitzyhere’s video of such run. When transitioning from Gold to Platinum he gained over 100 SR.
Why the system does that? It identifies him as a player of much higher rank. How does it do that? By stats comparison, it is the only way (I don’t think, that Blizzard implemented some hardware-based mechanism to determine who is playing).
If the system is able to analyze a player’s performance in a single match, it is able to do so retroactively. All in-game stats for an account are stored PERMANENTLY in a database on Bliz servers. It is accessible through the game interface. It would be foolish to assume, that all this data is not relevant, because in that case, why even bother gathering it and storing it, all you need for PBSR is actual data for a single match?
The following paragraph will be about ranks below diamond, where stats matter (simply winning games matters for 11% of total players - a sad fact about this game). Since OW is a game of large numbers, the matchmaker is designed in a manner to omit player’s peak performance moments. Player’s rank is not determined by popping off. It is determined by maintaining a certain winrate with certain stats for a large number of games. If you play rarely, matchmaker is not certain about your MMR (Scott Mercer quote, I believe), resulting in large SR swings. To the matter at hand. If your account has been played for several seasons for more than 100 games, matchmaker is certain about your performance. Your average stats are known. If you haven’t improved radically, your performance will not differ much from season to season. A quote from Kaawumba’s post (https://us.forums.blizzard.com/en/overwatch/t/how-competitive-skill-rating-works-season-14/278920):
This quote I can confirm as I can see stats for my new account, which is Plat right now. I maintain 52-55% winrate easily on this account. On my old account, which started out in Bronze, I barely maintained 50% winrate at low gold. I have improved, but not that much. But new account plays very differently, than the old one.
As for actual proof - I don’t know, what will be proof in this case. Probably, devs coming to this topic and labeling my assumptions as correct ones.