RSMB alternative

The unfortunate answer is there is no true equivalent, which is why RSMB remains popular long after AE included their "free" inbuilt Pixel Motion Blur effect.

PMB has extremely basic features, all you can do is control the global shutter delay. It can look OK at first glance for general live action footage where objects move across frame without changing shape (e.g. traffic in locked off shot) but the details are usually not physically correct. The way colors mix when several things move past one another is not trivial to calculate. PMB doesn't understand the difference between a foreground and background object, it just sees blocks of pixel colors changing and interpolates between them. Under the hood, PMB is based on the old Timewarp effect.

RSMB will look equally suspicious if applied as a simply click-and-hope effect, but it exposes a bunch of tools to fix the problems. You can fine-tune the blur direction for objects in the scene with masks, motion splines and track points, and for CG footage you can feed it 3D motion data from a velocity render pass. It takes time to teach it what's going on in the scene, but the results can be extremely accurate.

For SnapTuBook "I just want to make my drone footage blurry" clips the internal PMB effect is probably good enough, but to be blunt there's a reason it's free...