Tekken x Street Fighter is officially dead as confirmed by the game director and Tekken series producer Katsuhiro Harada. The game was announced long ago.
Tekken x Street Fighter project was announced alongside Street Fighter x Tekken. The concept at that time was that the Capcom side will create a Street Fighter x Tekken video game while Bandai Namco will work on a TxSF video game. The closest thing to this project was the inclusion of Akuma in Tekken 7, but nothing came out of this project.
In a livestream hosted by the Tekken series producer Katsuhiro Harada on June 5, it was revealed that the game is officially dead. Harada also revealed that only 30% of the game was completed and the project was put on ice afterward. While Harada wanted to show the game, it was not possible to do so. He didn’t reveal when the game was actually canceled but it should be noted that the Street Fighter series producer Yoshinori Ono has left Capcom a while back, and he was a crucial part of this project.
SFxT was a critical and commercial failure for Capcom and that could have contributed to the cancellation of the project. The game launched back in March 2012 and was released on multiple platforms. It failed to garner success and Capcom shelved any post-launch plans for the game.
If you wanted to see a 3D brawler featuring Street Fighter cast versus Tekken, your only hope is to get SFxT, which is only available on the PS3, Xbox 360, and the PS Vita. Aside from that, Akuma is actually a part of the Tekken canon which is pretty cool. He was a prominent character in Tekken 7.