I think the issue is that the latest API rating, SP I believe can only be met with synthetic or synthetic blend oils. Since so many consumers don’t even look at the API symbol, most retailers would not want to stock an older spec oil to prevent confusion and also reduce their inventory stock numbers.
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Except for Dollar General. Dollar General sued again for selling obsolete motor oil
You just posted an old article from 2018 
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In my experience, synthetics are fully compatible. I have gotten over 400K miles out of a '97 Honda D16Y71 engine that in the middle of its life went through a period of conventional and synthetic before ending fully synthetic. Everything I’ve read about synthetics say they are superior for lubrication and resisting breakdown. High mileage formulas can reduce oil usage, but there is no fountain of youth.
As for the API rating discussion, the latest rating is supposed to satisfy the standards of every rating that came before it, so there is no reason to shop for older ratings
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But, those high-mileage oils are designed to reduce seepage/leaking (by slightly swelling various seals), not to reduce oil burning. There are additives that claim to reduce oil burning, but I found them to help to only a very slight extent.
I wonder if there are any downsides to the “high mileage” oils? I’ve been using them in my cars for the past couple of years, not b/c I think HM oil is needed but b/c they are the oils with the brand & specs I want that the store routinely stocks. So far I see no difference at all.