It’s pure speculation. The rumour that Apple will ditch Intel for ARM in their Macs has been around a few years, but has stayed only a rumour.
I am unconvinced it will ever happen. Instead I can see them slowly make the iPad more Mac like. It already has support for a keyboard, a pointing device, split-screen multitasking. It is diverging from the iPhone with its own OS. Imagine if they expanded the multitasking support and added multiple display support, alongside more RAM to support more concurrent apps.
In which case you can keep making iOS/iPadOS apps with AIR. Add support for split-screen by letting your app work within a resizeable frame.
If it does change, and Mac apps need to include ARM code, I am sure Apple will give developers plenty of warning and help to do this. But, again, I am unconvinced it will happen.