My iPad Air model name A1474 / number MD785B/A has software 12.5.5 installed and is only really used when I am away from home but nothing works so what update should I be using?

The first generation iPad Air, iPad mini2 and mini3 can only be updated to iOS 12.5.6.

Apple ended major update support for these models of iPad in September 2019 - although they continue to receive occasional “point” security updates, the most recent being released August 2022. These devices cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements (CPU capabilities and available RAM) for newer versions of iOS/iPadOS.

Some App developers have dropped support for iOS versions preceding iOS 13.x for technical reasons (often because APIs or software libraries upon which they rely have been updated in the new version of iPadOS, making their App incompatible with earlier versions). 

Whilst many App titles remain compatible with your iPad, if you want or need to access to Apps requiring the most recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS15.

No, its not forced obsolescence at all. Apple maintains update support for as long as the device hardware can support the updated OS. The inability to update to a newer major version is typically due to inadequate RAM (older models of iPad may be limited to only 1GB or less - newer versions of iPad requiring at least 2GB or more) or a CPU that cannot support the new version.

Testament to commitment to maintaining devices as long as practicable is the security update issued only two months ago - for an aging model of over 8 years of age.

Storage capacity and RAM/CPU configuration are entirely unrelated…

An iPad that still receives updates - and still has a working battery - after 8 years is hardly a short lifespan. Most models of iPad have a lifespan of five to seven years, this being industry leading.

All computing devices ultimately have a support lifespan. A Windows PC will lose manufacturer support much sooner. Android devices may have a useable support lifespan of two years or less.