After a ton of testing, I came up with a very specific combination of settings in the Project Settings/Color Settings dialog. I was using HDR UHDTV Wide Color Gamut Display (Rec 2020) - E (presumably that E was for EOTF 2084).
I see nothing like that in the new version, uh oh! Any suggestions on getting those back, or what to use in its place?
Many of the color profiles were renamed in After Effects 14.2 to be clearer and use less complex names, favoring specific ITU specification names over long descriptive names. And specifically in this case, the old name was too long, causing it to be clipped by the UI's display limit.
The "HDR UHDTV Wide Color Gamut Display (Rec 2020) - E" profile is now named Rec.2100 PQ. Only the name has changed, the profile is the same as in previous releases of After Effects.