why do iPad apps I've used for 4-5 years suddenly require an "update from the developer"?

If the developer has abandoned that app or gone out of business and not updated in a long time, then eventually many such apps can break. Apple doesn’t give great details about the bug fixes or security patches or other underlying changes in any operating system version update.

So there is really no way of knowing why any particular old un-updated app stops working. Something changes in iOS that the old app’s code cannot work with. If the app is no longer supported, then there is typically nothing you can do but delete that app and try to find an alternative that is up to date. And the longer an app goes without being maintained and updated, the greater the certainty that it is going to become unable to load and run under a new updated operating system.

The nature of software licensing is that no developer has to provide support for their product after you’ve bought your license for it. Software is provided literally “as is” and “as available” and with no warranty or guarantee of any kind -> Legal - Licensed Application End User License Agreement - Apple. Any developer can just walk away from their product and pull it from distribution at any time they wish to.

For older iPad models with a Home button, do a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

OR

For much newer, larger screen iPad models, without a Home button, try the new hard reset procedure for the new 2018/2020/2021 iPad Pros and the 2020 iPad Air 4 models.

1. Press and Release the Volume Up button.

2. Then immediately Press and Release the Volume Down button.

3. Then, Press and Hold the Power Button at the top. You will see the Slide to Power off button, but keep on holding down the Power Button until you see the Apple logo, then let it go.

Once you have performed all the steps, wait for a few seconds and your 2018 iPad Pro will boot up completely to the iPad Pro’s Lock screen.

Best of Luck to You!

Since the transformation from iOS to iPadOS, I believe some apps had to be modified/updated by the developer to support the iPadOS functions, I would assume thats the issue there. Try finding alternate apps maybe?

Unless the app developer is willing to step up and help you, the contents are lost. It’s their app, so only they could do anything about it. If they’ve walked away from it, they likely won’t even respond to your query to them though.

I have the same problem and don’t mind replacing the app. I would like to access the the contents in the app. Is this possible? And how would I go about it? Or it possible to move the app to an older computer ( so that I can save the contents)? Or are the contents lost?