How to roll back an iOS update? Latest update broke apps.

Strictly speaking, it is possible to restore the immediately preceding [point] version of iOS/iPadOS - but only whilst the previous version is digitally “signed” by Apple.

After a “new” version is released, the immediately preceding release continues to be available for a few days - this providing a short overlap in time in which to identify a major/critical issue prior to revoking the signing-certificate of the prior release. As such, Apple ensure that “roll back” is possible during the early days of a full update.

After the update has been demonstrated to be free of major issues - that had not been identified during beta-test - the signing-certificate of the previous release is revoked by Apple. It is at this point the ability to restore the previous release is lost. You should note that the window of opportunity is not linked to the point-in-time that you install and update - but instead is a function of how long Apple are prepared to leave the opportunity open after a new release. This window of opportunity is typically days, not weeks.

You also need to understand that restoring the immediately preceding release is not a roll-back in the sense that you might perceive. Whilst the opportunity to restore the preceding release exists, the procedure fully reinstalls a fresh copy of the previous release - after which you are able to restore a pre-existing backup; the source backup must have been created from the restored (or earlier) version of iOS/iPadOS.

At this time, iPadOS 14.6 is currently the only signed (available) version of iPadOS. Apple revoked iPadOS 14.5.1 certificate signing on 1st June.

App compatibility (and subscriptions) are entirely under the control of the App Developer. The Developers business model is not controlled by Apple.

1. There is no way to roll back an iOS update.
2. App updates are free. There is no app update in the App Store that will cost any money. If the developers made a completely different app and are now selling that one too, that is a different thing all together.

1. If the update was corrupt, the device can be restored and the update can be applied again. If there was a major device breaking bug, I'm fairly certain Apple would immediately offer a way to fix it or if required go back to a previous version if it was deemed absolutely necessary. Rolling back is hardly ever a successful path to fixing an issue.
2. Most if not all apps that have subscriptions are strictly optional. Unless the App is now hiding features or options you want behind the subscription, it should be possible to use it without it at least in a limited or slightly restricted capacity. There is no requirement to actually pay for the subscription if you don't want to.