'My Journal' App no longer available

Then you have a problem…

Any Third Party (or Apple) App can be withdrawn at any time - and as such placing reliance upon long-term availability of a specific App to access critical data is unwise. This reality is not restricted to your iPad - but is true of any computing platform.

Unless you can identify and contact the Developer of the App with which you are having difficulties (and obtain their assistance with recovery of your data) - or unless you have previously exported important or irreplaceable data to an accessible archive - the likely unpalatable reality is that you have now lost your data.

While this new-found knowledge may not help you with this specific issue, you might be well advised to consider any other essential data that you have stored on your iPad (or other devices) - and consider your future data backup/archiving needs.

No 3rd party has ever come pre-installed on any iPad sold by Apple.

Who did you buy the original device from?

You will have to contact the app developer for support. That app is not an Apple product. It may have been abandoned by the developer or simply hasn’t been updated yet to support the current version of iPadOS.

iCloud will very likely have backed-up the data - but unfortunately, without the App that created it, this data is inaccessible to you.

Archiving or exporting your data in a non-proprietary format, that can be opened by other Apps, is your goal.

This App?

https://apps.apple.com/gb/app/my-journal/id1528552385