Not only vacuum friction can slow down light (in a STATIC universe) - the light source can also do so, as photons's journey starts:
"Spatially structured photons that travel in free space slower than the speed of light" Science 20 Feb 2015: Vol. 347, Issue 6224, pp. 857-860
"Physicists manage to slow down light inside vacuum [...] ...even now the light is no longer in the mask, it's just the propagating in free space - the speed is still slow. [...] "This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum," co-author Romero explains in the University of Glasgow press release."
"The speed of light is a limit, not a constant - that's what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently."
"Although the maximum speed of light is a cosmological constant - made famous by Einstein's Special Theory of Relativity and E=mc^2 - it can, in fact, be slowed down: that's what optics do."
"Glasgow researchers slow the speed of light"
So the essence of the "intrinsic redshift" idea - that the effect can be produced by the source of the light - has been unequivocally proved by experiments:
"Arp argued that the redshift was not due to Hubble expansion or physical movement of the objects, but must have a non-cosmological or "intrinsic" origin." https://en.wikipedia.org/wiki/Halton_Arp
"Expanding Universe "red shift" Myth debunked by Halton Arp using Quasars" https://www.youtube.com/watch?v=nV7JX9BZDMs
Needless to say, Halton Arp is an unperson in Einstein's schizophrenic world:
"Withers, however, was already an unperson. He did not exist : he had never existed."