The set of IDs (zones and links) in backzone is _almost_ a subset of the IDs in backward; the only ID that is in backzone but not in backward is Asia/Hanoi.
Can Asia/Hanoi be added as a link in backward to ensure compatibility with systems that use IDs from backzone? Is this a desirable invariant that can be upheld in the future? This was reported to CLDR (https://unicode-org.atlassian.net/browse/CLDR-11977), so it looks like there are systems that are using these IDs.
