Very, very unofficial. It was developed by the singular guy for 11 years with no help, feedback, critique or improvement of the old maps since. It's a GZDoom mod (not compatible with vanilla sourceports) with several GBs of size that will probably crash lesser machines on load.
There's been dozens of excellent community projects over the past 30 years that could reasonably be called "unofficial sequels", I don't know why this one got so much press.
In contrast, the new Doom 64 mod for GZDoom is really really cool and can turn most regular Doom custom maps INTO Doom 64 mechanics, especially the classic Doom 1 and 2 games. Definitely recommend checking out that one.