Your 1st gen iPad Air will no longer get any more support whatsoever from Apple.
However, 3rd party app developers will still continue to supoort iOS 11 and 12 for a few more years to come.
At some point, Apple's iOS version of Safari WILL not be secure.
You can switch to any third party Web browser that is still getting iOS updates that still supports iOS 11 or 12.
You will still be able to use that iPad Air 1 with nary an issue for the few years without any real worry.
There is absolutely NO reason to stop using the device entirely!
None.
Geez,
I am still using an 8-1/2 year old iPad 2 using a different iOS 9 supported Web browser.
Many, but not all, of my installed apps are still getting daily updates, to date.
Best of Luck to You!