Besides the light up plastic inserts... large clear acrylic sections are often used for things like "Basements".
Haunted House. (3 playfields: 1 Main level playfield. 1 mini upper level playfield. 1 mini under-playfield (basement)
HH Has a center mounted acrylic window that is transparent. Its dark, until the ball drops down into the basement level playfield... and then lights down turn on, allowing you to see everything.
On many playfields, they have Mylar glued onto the surface. Mylar is basically very thin plexi. It dramatically reduces ball wear. Without it... many machines have been worn right down to the bare wood... leaving actual pits. Also, heated lamps and aging materials often cause the wood to expand and contract.. and thus the paint and clearcoat (used to be some light shellac I think) ... would start to show tons of hairline cracks. Mylar seemed to help stop that from happening... due to the extra surface support it provided.
I believe I know of one or two machines that used plexi as an actual playfield topper. These were older machines... I think one was by Stern, called Orbiter. There might be many more. I think cutting costs nullified this idea... and they later decided to use mylar instead. In Orbiter is was a little different... in that theres a 3d molded plastic mountain range of sorts... thats under the plexi field.
Id thought about using a full 3/4 thick plexi or cutting board material, for certain similar projects.
Cost really jumps however, when you go to the thicker plexi. Also, not sure about how well things would work in pure plexi. If a post was hit hard.. it might crack. Glue doesnt always hold well either... though, you might not really need it, since the light can shine through your screen-printed artwork top from the underside.
As a surface for a ball, mylar is very tough.. and can last ages without wear. It can also be removed, and a new sheet re-applied if it ever does start to wear. Mild scraping can be buffed out quite easily, to a mirror shine. Its also very slippery when polished, which keeps wear even further reduced.
Do remember that Pinball playfields have a Lot of parts and thus a lot of weight on them. And pinball needs very accurate leveling... so the PF must be thick, dense, smooth as glass, and very rigid.
As for the ball.. many times it hits metal ramps & things, at great speeds.. and gets scratches on it. This ball then becomes a bit like a sanding machine... tearing up the playfield as it rolls and spins with fury. Its advisable to replace the balls every so often to help prevent quicker damages. Many use wax as well, to help reduce wear... by creating another layer of protection. I find that doesnt really work for very long.. and turns the PF into a real mess over time. I prefer to use F21 (Formula 21, from Turtle Wax) instead. Its tough urethane formula, provides a super slick surface... far slicker than wax.. and unlike wax... it cant be lifted off the field, nor store any particles of dust inside of it. Unlike wax... which is notorious for capturing carbon dust, metal flakes, and more.. into it.. and having the ball pickup, spread, and bash that dirty grime... all over the place.