Tod Howard recently confirmed that Bethesda will begin working on Fallout V after the completion of The Elder Scrolls 6, which is currently in pre-production: https://www.ign.com/articles/todd-howard-bethesda-fallout-5-elder-scrolls-6-starfield?utm_source=twitter

Will resolve to YES if the game is available for members of the public to purchase, download and play on any major console/platform, before January 1st 2031. Will resolve to NO if the game is only available as part of a "public beta" or similar.

If the game does not explicitly contain "Fallout V" in its title, I will resolve at my discretion whether it is still Fallout V de facto.