Seriously?? I'm not going to break down what each of the 10 subraces are, what they look like count artwork pieces in various WotC books or anything else you're asking for here.
I'm sorry. You said that I was basing my argument on assumptions, which certainly led me to assume that you had not. So let me help summarize some of what I found.
MToF: These eight tiefling subraces do not affect or mention any unique appearance of the tieflings. The subraces affect the secondary ability modifier and the tiefling bonus spells. All tieflings presented here have infernal ancestry. The art consisting of two tieflings is consistent with their art in 4-5e. The writers though do not spend much time (or page count) talking about tieflings in this book, as this section is predominately about Devils, Demons, and the Blood War. Dwarves, elves, halflings, gnomes, and gith get more discussion in this book.
SCAG: Gives the option for variant tieflings in a sidebar. There are four mechanical options. This is less subrace and more alternate racial features. The sidebar says that the appearance can be different, including more Planescape-familiar elements. However, we see no art of this here, and none appears in subsequent publications. Flavorwise, the tieflings are descrbed as having "the blood of fiends" and we are told that not all are of "the blood of Asmodeus." But all have the Mark of Asmodeus and they are described as being of infernal descent, even the non-Asmodeus ones. Their art (pp. cover, 119, 120, 128) is wholly consistent with the 4e art.
XGtE: I checked Xanathar's Guide, because it's the other book of player splat options and one that many players will see down the line. The tiefling art (pp. 13, 33, 37, 49, 56, 95, 127) is again more reminiscent of 4e and 5e PHB art for the tieflings.
Saltmarsh: The tiefling art (pp. 19, 49) are mostly consistent with 4e art. There is one tiefling (p. 92) that does not appear to have a tail. Does this tail-less tiefling represent the unbridled seething dissatisfaction and discontentment with the 4e-style tieflings?
This honestly suggests to me that tieflings have moved substantially towards their 4e incarnation than what they were in Planescape. They are infernal and mostly associated with Asmodeus foremost and other Lords of Hell secondarily. They have pronounced horns of varying shapes, human feet and legs, and a tail (apart from that one).
Or let's ask this question from another angle: What unique traces of the pre-4e Tiefling are in 5e Tieflings?
Again I am interested in the player base and whether they are choosing to play either in appearance or ability selection the base tiefling vs. the variants. Fan art, doesn't give me that info... the fact that WotC with the data they have (which I'm pretty sure is more rigorous than the amount of fan art from a google search)published alternate tieflings in not just 1 but 2 of the limited sourcebooks they've put out for 5e tells me that they were pretty sure there was demand for them.
And are they? But should we construe this as a demand for planescape-style tieflings or a dissatisfaction with 4e tieflings when they are just different flavors of diabolic tieflings? MToF basically just creates one tiefling subrace per ring of the Nine Hells. They're still diabolic, implied to have links with Asmodeus (all roads lead to Asmodeus in Hell), and largely depicted in appearance as they were in 4-5e.
And fan art you googled doesn't give us any data about the prevailing trend of what's actually being played in D&D... as I said earlier it's just silly, it could even be influenced by your browser history and preferences...
There are other websites with fanart, but I nevertheless suspect that your Google image search showed you much the same that mine did: a hyper-majority of tiefling art had them more congruent in appearance with 4e+ era.
