" I have not been able to find a product or sealant that would protect against yellowing. "

There is not one. Sunlight will effect dam near everything.

Engineered stone can't be stored outside uncovered for a reason, however, an occasional sunbeam in your kitchen isn't going to do much if anything.

How much sun are you getting that you think it will be an issue?

Actual white stone shouldn't have issues. Outdoor quartz for sure shouldn't, since it's made to withstand outdoor sun. Good quality quartz might still work depending on how much sun you are getting. With white quartz, it isn't really the quartz itself that can yellow, it's sometimes the pigments and more often the resins that can go off.

Other things can cause white quartz to yellow too. Cleaners, spills, heat- just some regular use can buld up. So even if it's UV ok, you will still need to care for it to keep it at it's whitest.

If there is strong UV in the space then quartz like every other thing will be affected The simplest is to do solar shades on the windows that face the sun , the control UV and also allow you to keep the view. They are not 100% effective but certainly help. If you are concerened why not look at real stone instead of fake plastic. The real stone has been exposed to the elements for a very long time.

... but be careful, even "real stone" might be stabilized with "plastic" ...

As others have asked, how much sun are you getting in your kitchen? If it is a lot, and since you are concerned about the Quartz yellowing, what are you doing about your cabinetry? If the cabinetry is a wood stain, strong sun exposure will also change its color. If the cabinetry is a painted finish, strong sun exposure will also change its color.

It seems to me that by looking for a coating for the counter, you are not addressing the problem, which extends to your cabinets. What about installing light filtering blinds on your windows?

or the home is from 1920

I agree with what you are saying about windows and UV blocking, but not the 1920 year. My home had 1991 Pella windows and there was no LoE coating on them. I've replaced those windows since then. I think the LoE technology started approx 1980, but did not become a widespread application until the late '90s/early 2000s.

If the OP does not have any UV protection on her windows, but does not want to spend tens of thousands on all new windows, I reiterate my suggestion above for light filtering blinds.

I have a sunny kitchen with multiple windows which is why I'm concerned. My preference is real stone, A white Marble, the real deal which has it's own set of issues. But over time, the patina of marble is fine with me because it's a natural stone. Every fabricator I visit also have a showroom of quartz samples from multiple manufacturers and try direct me away from marble. I wonder if it's self serving. they make more money from selling quartz then as a reseller of real stone. Just an assumption.

LOL, My home is from the 1930's. We painstakingly restored the single pain wood windows but replaced the glass. I'll have to ask if they have a LoE coating.

1930 home, no uv coating on your windows, get the real marble (after reading all the threads) I direct most of my clients to estone but it is not the same as real stone (obviously)

I am a fabricator. " They are trying to direct you away from marble because many people are not prepared for marble's etching, staining and softness. " This is exactly right. If a customer insists on marble in the kitchen we require a sign off that notes the material properties and the certainty of scratches, etching, and stains. Even then some customers think they have legitimate cause to be unhappy with the fabricator when those exact things happen, usually within a few weeks of install. The only time I am comfortable with marble in the kitchen is when the customer has had marble in a previous kitchen and knows what they are in for.