Let’s see. I was buying beaters from junkyards back in the 80s. You’ll notice I said beaters,plural. They weren’t decent, not unless you consider a brown car with one green door and a lot of dents “decent”. Only one of them ( the one that lasted a couple of months) was $100. The others ( there were 3 more that I paid for ) cost anywhere between $250 and $500
True, my first vehicle was a '65 Ford F-100, that needed brake work (less than $35 in parts to fix those, another $35 for an engine mount, and about $25 to add seat belts). Bought for $200. Worked fine for the year and some change that I drove it before I rolled it (my fault, not a failure on its part).
It would probably still be trucking along if I hadn’t rolled it. That 300 ci six was pretty indestructible. But it still was about 3x $100.
I don’t think it necessarily has to be high end to be a luxury nor does a luxury have to be high end. If I had two cars now it would be a luxury - but it wasn’t while we were both working. My son has a 2013 Honda CRV - he paid $13K for it. Still a luxury because he hardly ever drives it - he lives in Manhattan and parks it near my house so he only takes it when he wants to go on a weekend trip or to the warehouse store.
A car (or a truck) is a necessity for most people, most places. That’s why I was pondering calling it a “luxury”. They might have luxury prices now, but that’s something altogether different.
Right- and I’m saying the price really doesn’t have much to do with whether it’s a luxury or not. It’s the circumstances.
Plastic don’t rust. 
I remember when I was shopping for my last new car, my mechanic warned me, “Be careful what you choose. This will probably be the second-last car you ever buy, so you’ll want to keep it a long time.”
That was in 2014. The car now had 80,000 miles on it, and the way my wife and I are going it will very likely be the last car we ever bought.
Maybe it is an affordable luxury because of artificially low fuel/energy prices. I will bet you are not paying $15/gallon. Yet.
There’s a sense in which using a personal car or truck can be said to have a “luxury” aspect irrespective of what it costs. Namely, for most drivers, there’s a substantial proportion of trips where the potential capacity/performance of the vehicle is wildly disproportionate to the more limited requirements of what it’s actually being used for.
E.g., it’s intrinsically, functionally wasteful for an able-bodied individual to drag along about 1500 kg and 12 cubic meters of climate-controlled, multi-passenger, high-speed-enabled vehicle with substantial cargo capacity just to transport their own body (and maybe an easily portable bag) a couple-few kilometers in a physically tolerable environment.
Nonetheless, that wasteful setup is some combination of culturally-mandated and logistically-necessary for millions of Americans every day of their lives. Either they don’t have workable mass transit alternatives, and/or they’re not physically capable of (or don’t have safe roadways for) micromobility transit modes like walking or bicycling, and/or their schedules are tight enough that they can’t spare an extra half-hour or so a day for transportation, and/or if they have to own the car anyway for other purposes then there’s no point spending extra to use a different transit mode for short trips, etc. Or they’ve just never seriously thought at all about possible strategies for getting around without owning a car.
Consequently, dragging a whole entire car along with you whenever you need to go anywhere (and maybe even having to pay to park it once you get there) certainly doesn’t feel luxurious to most drivers. But when you stop to think about it, there’s no question that a certain amount of functional extravagance is built in to the American commitment to the private motor vehicle for the overwhelming majority of our transportation. If constantly using far more resources than a task physically requires counts as “luxury”, then most privately owned cars in the US qualify.
Maybe my title was a bit too abbreviated, but I did narrow the definition in the OP:
owning a reliable, well-maintained and well-repaired vehicle is becoming a luxury.
The idea that there are fewer reliable and well-maintained cars on the road (which is what you’d see if they were becoming a luxury) doesn’t pass a sniff test to me (maybe if you used a really recent baseline for comparison, like 2019 or something), but thankfully there are lots of statistics about this kind of thing.
I’ll look at the poorest fifth of households, since that’s surely where this would have the biggest impact.
In 1999, households in the lowest quintile of income spent 16.7% of their household expenditures on transportation, and 8.7% of transportation expenditures (so 1.5% of all expenditures) were on maintenance and repairs. These households had, on average, 1.0 vehicles per household, and 34% of them had no car.
In 2024, the most recent year that’s available, transportation as a share of household expenditure was down slightly to 14.6% in the lowest quintile, and maintenance was also down a bit as a share of transportation expenditures to 7.7% (1.1% of all expenditures). They still had 1.0 vehicles per household, with 32% having no car.
So, the poorest fifth of households are just about as likely to have a car as they were a quarter of a century earlier, and they’re using a slightly smaller share of their outgoings on keeping their cars running.