Unfortunately, this is true, then.
You will need to purchase a new iPad or a newer, refurbished iPad from Apple’s refurbished store, or look at something newer on the used market, making sure you do not get/purchase an iPad with Activation Lock.
Look, you have had a 6 year old iPad that got one more iOS upgrade than most.
Any technology is going to move forward and with mobile phones and mobile tablets, technology moves ahead much faster.
Android won’t be any better and you WILL have to be careful of the thousands of malware/viruses that are part of the Android experience.
My wife and I still own iPad 2 and 3 models and they haven't had any iOS upgrades in over 2/12 years.
We still get everyday use from these devices.
The iPad 4th gen only came out 6 months after the iPad 3 and my wife’s iPad 3 didn’t get iOS 10.
So, your iPad 4th gen got a whole additional year of iOS 10 upgrades/updates.
Yes, you could get a cheap Android phone or tablet to get access to these currently updated airline apps.
You can even purchase an iPhone SE from carriers, like T-Moble/Family Mobile and a couple others on the cheap now that supports iOS 11 and iOS 12.
My Wife and I own iPhone SE 32 GBs storage models. We paid only $160 each for these and use a fairly cheap “pay as you go” plan.
Your iPad 4th gen is still very useful for other things beside booking an airplane flight.
The decisions are strictly up to you.
Good Luck!