Is the iPhone 8Plus waterproof?

No, it is water resistant, but by no means should be considered waterproof.

No iPhone model is waterproof. The warranty is voided by water damage on all iPhones.

No. It has some degree of water resistance. Which means it can take some light splashing , dust etc...

It does not mean it can be submerged in water, nor should it be used near a pool or bathtub.

Read here: About splash, water, and dust resistance of iPhone 7 and later - Apple Support