Apple has said they will support older iOS and iPadOS devices which are too old to update to 15 with the recently released 12.x. That likely means Apple will periodically release security updates only. Don't expect any new features on an older version of iOS and iPadOS.
Supposedly Apple will provide security updates for iOS and iPadOS 14.x, but again, don't expect any new features.
It is in Apple's best interest to keep older versions updated with current security updates. Otherwise they could wind up with a mess. However, Apple is hardcore when it comes to trying their best to FORCE us to update to the latest version. They don't want to have to update and maintain older versions, that is expensive, so the quicker they force the latest version on us, the sooner they can obsolete the older versions and stop providing security updates.