You have no cause for concern over the apparent jump from iPadOS 18 to iPadOS 26.
With the new releases of Apple's various Operating Systems in September 2025 (for iOS, iPadOS, MacOS, TVOS, VisionOS etc.), Apple re-aligned all of its OS numbering to the Calendar year. As such, for iOS/iPadOS, the major version number simply stepped from 18.x to 26.x. You have not missed any intermediate versions.
At the time of writing, for iPad devices that are still running iPadOS 18.x, you can either (a) update to iPadOS 18.7.3, or (b) upgrade to iPadOS 26.2 - both being the most recent releases for these two major versions.
While you currently have the option to maintain your iPad at iPadOS 18.x with available security updates, it won't be long before update to iPadOS 26.x will be necessary to maintain security updates for iPad. For iPhone and iOS, for devices capable of running iOS 26.x, security updates are now only available by upgrading to iOS 26.2.