Yeah it appears the bitfield is gone.
And unfortunately there's no flyability filter. That would be an awesome addition for users. Whether a mount can fly or not is usually a major choice in grouping sets of mounts.
Here's all the filters:
Tooltip scanning is an odorous last resort option. It's slow as death on crutches and requires localization. I need to do some testing on live where we can tell if a mount can fly, but all flying mounts' tooltips seem to end in this string:
"This is a flying mount."
The capabilities of this mount depend "on your Riding skill and location."
This mount changes depending "on your Riding skill and location."
(Land mounts that change depending on riding skill don't mention location; they end with "on your Riding skill.")
From scaning tooltips (TextLeft3 and 4 for the above two strings) it only hits 156 out of 455 mounts that can fly. That number seems low so could be other strings too.
Probably if we don't get a return to tell if a mount can fly, our addons can use a huge table of all spellIDs that can fly. It wouldn't require localization or client-halting tooltip scans, but the table would need updated every time they add a new mount.
But yeah, getting the bitfield back or even a true/false whether a mount can fly would be awesome.