Silk screening has been around for a long time (about 1000 years in China has a form of screen printing).

Screen printing involves creating a screen (stencil) and then using that screen to apply layers of ink on the printing surface. Each color is applied using a different screen, one at a time, combined to achieve the final look. Most playfields have 5-12 different screen layers and it is a timely process.

Digital printing is a much newer process that involves your artwork being processed by a computer, and then printed directly onto the playfield surface.

Screen printing is the best option for designs that require a high level of vibrancy and custom colors. The ink in screen printing is applied thicker than digital printing, which results in brighter colors.

Digital printing is best used for items that require high amounts of detail, and orders of a smaller quantity. The fact that the digital printer does not use screens allows for a photographic print, with much more detail than traditional screen printing. As the ink is applied thinner (to achieve such detail). The fact that the design is processed and printed digitally allows for a quantity of one, since there are no screens or physical setup.

Do modern playfields use C-M-Y-K when they are screen printed?

https://classicplayfields.com/

Yes 100% look at William's games then look at modern sterns.
You can see the printing lines on modern stern playfeilds if you look closely.

Maybe. It depends on the playfield title and the quality of the print.

There are advantages to digital printed playfield, like registration. Silk screen playfields can have a slightly shifted registration between the different screen printed colors. Some silk screened playfields look bad if the registration is too shifted.

Basically more and more playfields are being digitally printed now. Some look good, and some don't.