Is this Asbestos

A membrane (whether thin sheet or liquid) applied over the backerboard, is a vastly superior means of waterproofing, This is because it sits immediately under the tile, as opposed to behind the backer.

Red Guard and HydroBan are some liquid membranes and Kerdi and NobleFlex are thin sheet membranes.

AFAIR, asbestos mastic is much more likely with tile (which may itself contain asbestos) - I would not expect it under carpet, other than as a remnant of a prior tile job (no sign of that here.) And, as you have noted, it doesn't look like it.

While you could engage full paranoia (as the folks that make money from it like) and have it tested, you can probably remove it without too much concern, and if it's "gummy" you might want to take a "solvent and wipe up" approach anyway, which would minimize anything getting in the air.