  The coincident detection of gravitational waves (GW) and a gamma-ray burst
from a merger of neutron stars has placed an extremely stringent bound on the
speed of GW. We showed previously that the presence of gravitational slip
($\eta$) in cosmology is intimately tied to modifications of GW propagation.
This new constraint implies that the only remaining viable source of
gravitational slip is a conformal coupling to gravity in scalar-tensor
theories, while viable vector-tensor theories cannot now generate gravitational
slip at all. We discuss structure formation in the remaining viable models,
demonstrating that (i) the dark-matter growth rate must now be at least as fast
as in GR, with the possible exception of the beyond Horndeski model. (ii) If
there is any scale-dependence at all in the slip parameter, it is such that it
takes the GR value at large scales. We show a consistency relation which must
be violated if gravity is modified.
