My iPad Air is on iOS12.4.9 and does not update to later releases. It says the software is up to date. Certain apps will no longer work. Why is this

If the iPad tells you that the software is up to date, it is. Your iPad has reached its limit for iOS updates. Newer apps that require iOS 13 or higher will not work on the device. If you want to run newer apps, or apps that have been updated past iOS 12, you will need to purchase a newer device.

Your over 7-year old, 2013 iPad Air 1st generation cannot upgrade beyond any version of iOS 12, currently at iOS 12.4.9.

Your iPad model is too old, now.

Sorry.

If you need/want an iPad with an up to date iPadOS with access to current, updated apps, then you really need to seriously consider a new iPad model OR a much “newer” , used iPad model capable of running the latest iPadOS versions.

There are plenty of newer, older iPad models that can run the latest, current versions of iPadOS.

Sorry & Best of Luck to You!

Indeed. Merry Christmas.