  In four-dimensional asymptotically flat spacetimes, an infinite tower of soft
graviton modes is known to generate the symmetry algebra of ${\rm
w}_{1+\infty}$ at tree-level. Here we demonstrate that the symmetry action
follows from soft graviton theorems and acts non-trivially on massive scalar
particles. By generalizing previous analyses that were specifically tailored to
the scattering of massless particles, our results clarify that ${\rm
w}_{1+\infty}$ symmetry is a universal feature of tree-level gravitational
scattering in four-dimensional asymptotically flat spacetimes and originates
from minimally-coupled gravitational interactions. In addition, we show that
the ${\rm w}_{1+\infty}$ symmetry acts non-diagonally on massive states by
mixing an infinite number of conformal families. We also present a concrete
example of non-local behavior on the celestial sphere in the presence of
massive scattering states.
