yes, the speed of light (photons, EM & gravity waves...) in reality is less than the constant c = 299792458 m/s
The constant has this name but it is a limiting value.
It *would* be the speed of propagation of these fields, if the classical vacuum would exist,
but it does not exist.
In reality one has L = c / N with refractive index N > 1.
But to say "light has energy and because of E=mc² it also has mass"
is a slightly dangerous statement / confusing reasoning,
which I see very often on internet.
One has to distinguish the "mass at rest m0", which is a parameter,
from the mass which is the source of gravitational field, inertia, etc.
It is a similar problem as "speed of light" : the same word / expression
has two distinct meanings.