Will the iPhone 7 Plus be able to upgrade to iOS 18?

Never. An iPhone 7 cannot be updated beyond iOS 15.8.3. It doesn't have the hardware needed to run anything higher.

If you need to run a higher operating system, you will need to purchase have a newer phone.

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8.3

• If you have an iPhone 8 or X, you can update to 16.7.10

• If you have an iPhone SE (2nd generation or later), or XR through 15, you can update to 17.7 or 18.0.1

An iPhone 7plus cannot update past iOS 15. If you need the current version, you will need a newer iPhone.

No. The best that the iPhone 7 Plus can ever do is iOS 15.8.3, so this phone is 3-4 years behind at this point.