The voice actor of Deacon from Fallout 4, Ryan Alosio, let the secret of Fallout 5 slip sometime in 2016. I have posted a different article on this subject it I had almost no info on the subject
I have found out that Bethesda launched post launch DLC campaign tha started in the later part of 2016 and is still going here in 2017. I recently found a verified screenshot of Alosios Instagram post that started the rumors of Fallout 5's production. The exact words that started the chaos were, "...internaly, there is movement gearing up pre-production of FO5 (shhhhh)" as well as, "There's so much cool stuff coming up in the Fallout universe..." Ryan Alosio still denies his words saying "I was just being in character." Some of my other sources are saying that Fallout 5 is being confused with the new Fallout VR but I have found that, that is very unlikely. Comparing the time it took for Fallout five to come out after a different Fallout 3 voice actor let a secret slip of Fallout 4 and the time before Bethesda gave the real confirmation, we may not see the actual all clear until 2018 and the game itself may not even come out until sometime between 2019 and 2020.