Last point first - iPadOS16 is not due for release until sometime in October. For reasons known only to Apple, release dates for iOS for iPhone and iPadOS for iPad have been deliberately staggered.
However, iPadOS 15.7 was released 12th September - incorporating essential security updates for iPad.
About the security content of iOS 15.7 and iPadOS 15.7 - Apple Support
When iPadOS16 is released, it will be available for these models of iPad:
- iPad Pro (all models)
- iPad Air (3rd generation and later)
- iPad (5th generation and later)
- iPad mini (5th generation and later)
More information about the forthcoming iPadOS16 can be found here:
iPadOS 16 Preview - Apple
Returning to your issue, iPad is generally very efficient with its memory and resource management - but sometimes does require a little help by manually closing dormant Apps. Older models of iPad are already relatively resource-constrained; while Apps may be technically compatible with older models of iPad when running current versions of iPadOS, performance and stability may be an issue.