Given that the M1 is just like any other Apple processor made for any of their other devices, it's easy to figure a rough estimation.
The M1 is based off of the same cores as the A14. It was also last used in a device introduced in March 2022, the iPad Air, with the A14 last being used in the iPad 10 introduced in October 2022.
Apple always gives the base iPad seven full iPadOS updates, likely due to the education market it is targeted at. This is why sometimes there will be weird gaps like the A10 iPad 7 getting iPad OS 18 but the A10X iPad Pros only getting up to 17. So the A14 based iPad 10 will at least get to iPadOS 29.
iPadOS 26 supports every iPad introduced from October 2018 and later, including the March 2019 Air3 and Mini5. This is a very good sign, it means that devices like the M1 Air will likely be supported right alongside the iPad 10’s A14 until iPadOS 29.
We can assume If a chip is supported on iPadOS, macOS is supported as well.
macOS 27 is already known to be M1 and up, the first ARM only release.
Given that Apple is literally still selling the M1 MacBook Air through Wallmart, it’s a pretty good chance macOS 28 keeps support as well. And given what we already established about iPad support, macOS 29 support is very likely as well.
With this in mind, the M1 being supported until iPadOS and macOS 29 means that the MacBook Air will get eight full years of software support, with the rest of the M1 family from 2021 and 2022 getting seven and six full years respectively, plus the additional three years of security updates.
macOS 27, 28 and 29 are all very likely to have the same system requirements just like Mavericks, Yosemite and El Capitan did.
TLDR: macOS 28 is a pretty safe bet, with macOS 29 being very likely as well.
