Options for raised bathtub platform
No slope is a bad idea - sure, you can have the joy of vacuuming up water on the platform after every shower or bath...no, really you want drainage.
If the tub (as usual) stays put, you can set the tub level, and slope the surface around it (or under it, other than where the feet sit, if it has feet) to drain.
Honestly, my first thought here is that the tub base/surround should be a non-raised extension of the shower base. A step like that invites falls in the bathroom. You step out of the tub, it's further down to the shower floor than your brain expects, or you slip off the narrow step, and hurt yourself.
It should still be sloped to drain, while the part exactly under the tub should be level (and full of plaster, since I'm old-school that way with making plastic tubs be solidly bedded. Tubs that flex, crack.) Either raise the side towards the drain to be level, so the tub sits on a very short plinth, or let the surround base &amp; tile flow up the skirt of the tub to embed it the half-inch to an inch it might need on the high side.