HDMI 2.0 officially supports 120hz at 1080p so there is no reason for any projector to not accept it.
To encourage more manufacturers to do so, and help users out who are like me, I'm compiling a list of 4K Projectors with support for 120hz for gaming, 3D or true high framerate (HFR) enthusiasts.
120hz in 2D also theoretically opens the door to unofficial frame sequential 3D support, if you can sync LCD shutter glasses from your PC via IR transmitters available online. This will be 60hz per eye 3D, so Bluray 24p framepacked content will either be shown using 3:2 pulldown judder or you have to do frame interpolation on your PC. It's also possible to sync at 96hz (48hz per eye), using 2:2 pulldown but that remains to be seen. Users would have to add a custom 48hz mode at 1080p and then enable 3D display, if 96hz doesn't work outright. There's a good chance it will, but few seem willing to try it.
Here's the list so far, I will update this post as more become confirmed.
Note, it's not enough for the projector to state in its manual it supports 1920 x 1080 x 120hz, users should make sure it's really doing true 120hz by checking TestUfo's FrameSkipping test, that it doesn't miss drawing any squares:
BenQ HT2550 / W1700:
Recommendations for Gaming: pass on these for now.
The BenQ has 600:1 contrast ratio but 50ms input lag, while the Acer is 1900:1 and laser + FI + WCG, but 80ms input lag even with FI disabled. Bummer.
Also while the 2X XPR models (0.66 inch chip) have twice the number of mirrors and thus resolution, you cannot input at the native resolution (2716 x 1528) which means 1080p 120hz will be smoother but not as sharp as it could be. Therefore for 1080p content you're better off with a 1080p native display and thus an XPR 2X or eshift projector.
Perhaps Epson or JVC will release a 120hz 1080p mode to their 2018 / 2019 projectors for frame sequential 3D at 1080p60, which is the same thing as 1080p 120hz in 2D.
The ultimate for me would be a decent contrast 4k (via eShift or quad XPR) capable of 1080p / 120hz + HDR + WCG + Laser + Low input lag for gaming.
The Epson 5040 has over 5000:1 native contrast ratio so even an update to that model with true 18 gbps HDMI 2.0 chips could enable 120hz at 1080p for gaming later this year hopefully. Their low lamp costs, high lumens for 3D, low input lag combined with fairly high contrast (compared to DLP) makes them appealing. Crosstalk in 3D is worse than DLP but for 120hz 2D it's likely better, since the difference between two frames in 2D is likely less than left and right eyes in 3D, which is what LCDs would have to transition between 120 times a second.
There's also a good chance that other XPR DLPs will add support for 120hz input via firmware update. There is literally no reason they can't, except for lack manufacturer will. 120hz 1080p is officially mandated in HDMI 2.0 and all of these projectors are capable of 120hz internally already.
Let's do this!