When a product says ....”item supports iPhone 8 and later” am I to assume that it does not support iPhone 6Plus?

It would not support the iPhone 6 Plus.

Correct. If is says "iPhone 8 and later" it means the iPhone 8 and other iPhones released after the iPhone 8.

The iPhone 6 and 6 Plus would have been released before the iPhone 8.

You are welcome.