The 2014 version of Thanos knows who the Avengers are in Avengers: Endgame, but how could he? A clash between the Mad Titan and Earth's Mightiest Heroes has been building in the Marvel Cinematic Universe since the end of The Avengers, when Thanos first made an appearance. It wasn't until last year's Avengers: Infinity War that this fight really began, and audiences were ultimately shocked that Thanos come out victorious.
In Avengers: Endgame, the remaining heroes quickly kill Thanos after he destroys the Infinity Stones, and the only way for them to undo this is through time travel. This plan comes with a few complications with the Avengers creating some branched timelines along the way. Most dangerously, when Nebula's memory network is connected to her past self, she shows a 2014 version of Thanos the future and his ultimate fate.
Once he scans 2023 Nebula's entire memory for himself and Infinity Stones, he stumbles upon a recording of Nebula with the Avengers. Gamora recognizes them as simply Terran, but, Thanos has a more clear understanding of who these people are, referring to them as Avengers. But how did he recognize them so immediately.
It's true that Thanos was behind the events of The Avengers and was informed of their victory over Loki by The Other. In fact, in Endgame he even mirrors his former henchman's words, describing them as "wretches." That isn't as satisfying an answer as many would like, but it is the one that makes the most sense. Whether that knowledge comes directly from The Other or through some other means is not clear, but Thanos knowing enough about the Avengers to identify them does line up with the rest of the MCU canon.
One major moment from Avengers: Infinity War revealed that Thanos knew who Tony Stark was long before they met on Titan. The Russo brothers explained that Thanos is aware of who Tony is from the Battle of New York as he single-handily wiped out his Chitauri fleet. This explanation only directly accounts for his knowledge of Tony, but it could go further; or, if 2014 Thanos only recognized Stark, he could have pieced together that the team were Avengers.
This admittedly doesn't explain details like 2014 Thanos knowing the team call themselves the Avengers, what each person looks like, and what their names are. But, until a true explanation for 2014 Thanos recognizing the Avengers in Endgame comes or a future MCU film addresses it, this is possibly the best solution.
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