How do I get rid of this annoying prompt...and what happened to the Pantone color books?

Here's my take: all a theory, of course.

Pantone was relevant when people were actually using their inks. Any printer using Pantone colour, had to buy their ink from a Pantone-licensed ink supplier, or buy the set of Pantone base inks that were then used to mix each of the colours in house. That really WAS their bread and butter in those days. Printers weren't even charged for swatch books, as long as they used their ink. Others (like agencies and designers) would have to pay for them mostly because they are hella expensive to print. In my years in the print industry did we ever pay for swatch books.

As more CMYK work and alternative printing processes starting replacing the use of actual spot inks on the press, designers literally stopped opting for spot inks due to the expense of it. At the printer I worked at, we saw less and less Pantone jobs over the years, even from the bigger Agencies. Yes, they were still needed and used for single or 2-colour offset print jobs, and specific brand work, but fewer designers are designing 5-colour or 6-colour print jobs these days. Even to them, digital is cheaper, and better than ever.

Hence, as many printers become more digitally-based, Pantone was becoming less relevant, and although it is possble to do relatively good matches to Pantone inks with wider gamut printers, why even bother. And, of course, this is not good for Pantone as they are no longer selling as much bread and butter ink.

Hence, I suspect that Pantone saw a need to improve a revenue stream, and they started moving to their subscription-based systems as one way to do that. (I personally hate that approcah.... you wouldn't like having to pay a monthly fee to keep an Ikea catalog on your shelf!) I suspect the licensing fee to include them in apps were going up as well, and this is probably where the beef with Adobe started... similar to the decision to drop licensing and reselling of Linotype/Monotye fonts. Even before they were deleted form CC apps, Adobe hadn't updated the libraries they were supplying, missing many new colours. Yes, Corel and Affinity and Quark still have them, but it's because they are likely still willing to pay Pantone to license them, relevant or not....   for now..... who knows!

I see it in Photoshop at the bottom of the Window menu: Contextual Task Bar.