Apple apparently won't bring iPadOS 18 to three existing models that
currently have iPadOS 17.
https://www.techradar.com/tablets/ipad/apple-reportedly-wont-bring-ipados-18-to-three-ipads-but-theres-better-news-for-iphones

The other iPads currently running iPadOS 17 should get full updates though,
meaning that if you have an iPad, iPad Air or iPad mini from 2019 or later,
or an iPad Pro from 2018 or later, you should get at least one more major
software update, according to this leak.

We would however take this with a pinch of salt, especially since, as
9to5Mac notes, the two Pro models that apparently aren't getting iPadOS 18
have an A10X Fusion chipset, while the oldest standard iPad that's said to
be in line for the update has a less powerful A10 Fusion chipset, which
seems odd.

This leak also included information about the phones that will apparently
get iOS 18, and it's better news there, as apparently all the iPhones that
can get iOS 17 will also get iOS 18. That means the iPhone XR and iPhone XS
onwards, as well as the iPhone SE (2020) onwards.

We've heard the same claims about iOS 18 compatibility before, and hearing
them again from another source adds credibility to it. That older leak also
mentioned iPadOS 18 compatibility, and similarly said that iPads with the
A10X Fusion chipset wouldn't get it.

Apple has a long standing iron clad published policy that Apple only fully
updates only one iOS release (and only one macOS release) so now is the
time to buy a new iPad or iPhone if you have those that won't update.
<https://support.apple.com/guide/deployment/about-software-updates-depc4c80847a/>

Therefore, if you've been holding onto an older iPad that will no longer
update to iOS 18, then this is the year to upgrade, as not only are we
expecting to see the iPad Pro 2024, the iPad Air 6, the iPad 11, but also
the iPad mini 7.