What to reproject when doing temporal filtering in stochastic screen space reflection?
Usual reprojection finds UV of current visible point into previous frame. For diffuse and rough objects it simply adds motion vector: vec2 UV_reprojected = UV + motion;
But for mirror-like surfaces it doesn't work, because we see not object (mirror) itself but what reflects in mirror (virtual object). The general idea to reproject reflected point is:

Find world space position for reflected point visible in reflector (Here we need to know reflected path length).
Substract world space delta between frames, so we found previous world position.
Project this point into reflector using previous camera position.
Project world space reflector point back to the screen.

The most hardest part is p.3, and is not easy analytically find this point, because reflector can change normal. Some interesting links about this topic:

Reflection reprojection in Uncharted. Here we assume that reflector is plane that changes transform, both camera and receiver are also dynamic.
Ray Tracing Gems chapter 32. (32.4.3) Theoretical approach that finds reflector position in terms of least squares. And interesting geometrical optic approach (here we assume that reflector point is locally sphere-like).