Sometimes an existing example is modified to show a new feature, e.g. #23208. Would you still add the tag in that case?

I think so - it's still showing a feature that came in recently. I think the only complication is if an example warrants multiple version tags, making it clear which feature goes to which tag - but that may be a good usecase for putting tags in places other than the bottom if it's an example with multiple parts.

IMHO the Tag should only be set if the feature is the central aspect of the example. It should be rare that we‘ll have duplicate labels then.

More generally, labels can have two functions:

• Show the minimal requirement needed for the example.
• Draw attentions to new features.

The first one is not reliable, because minor aspects of the example may still need newer versions.
This leaves us with the second, so that we can always resolve to the newest version.

the Tag should only be set if the feature is the central aspect of the example

I'm fine w/ that criteria and it fits w/ the other "versioning guidelines" criteria.