I am testing this on version 2.3.4, the "Craftoria" modpack on NeoForge.
So, in earlier versions, if you hold an empty barrel at hand and left or right click on another barrel, you could take or give stacks from it respectively. This feature doesn't seem to work anymore. The stacks get added to the inventory instead. When trying to give a stack from a full barrel, it transfers the barrel itself, which is not very useful, to say the last.
Is this a bug or an intended change?

This will be for the 1.22 release.

Any way to add this to 1.21 as well?

Any way to add this to 1.21 as well?

Given the changes in 1.21.10 it's better to wait for 1.21.10+, otherwise I'd have to do it twice (completely differently in 1.21.1).