Ping translatewiki.net staff here to see if they know why/hows/etc.

This language has not been exported for a long long time. The history goes further than the beginning of version control in a66708c6 in 2012.

To me it looks more like an omission from this list in TranslateSettings.php:

'zh' => 'This language code should remain unused. Localise in zh-hans or zh-hant please.',
'zh-classical' => 'This language code should remain unused. Localise in lzh please.',
'zh-cn' => 'This language code should remain unused. Localise in zh-hans please.',
'zh-tw' => 'This language code should remain unused. Localise in zh-hant please.',
'zh-min-nan' => 'This language code should remain unused. Localise in nan please.',
'zh-mo' => 'This language code should remain unused. Localise in zh-hk please.',
'zh-my' => 'This language code should remain unused. Localise in zh-sg please.',
'zh-sg' => 'This language code should remain unused. Localise in zh-hans please.',
'zh-yue' => 'This language code should remain unused. Localise in yue please.',

The files that already exists should probably be removed.

That is assuming that nobody can explain why this variant should exist in MediaWiki.

@Nikerabbit I don't think simply disable would be helpful because in https://translatewiki.net/wiki/Thread:Support/Enable_syncing_of_zh-tw_and_zh-hk_for_MediaWiki#Enable_syncing_of_zh-tw_and_zh-hk_for_MediaWiki_41700, @Jdforrester-WMF says:

There are real language differences between zh-TW and zh-HK in the MediaWiki files. For example, the word for 'bot' is '機器人' in Taiwan but '機械人' in Hong Kong. Note that both are written in Traditional Chinese (the Simplified Chinese equivalents would be 机器人 and 机械人).

Disabling properly wouldn't change the status quo. So, there are differences between the two variants. It's still unclear to me

1. Who are the users who would use zh-Hant and zh-HK respectively?
2. Do we have translators willing to maintain zh-HK and understanding that overrides should only be done if the translations differ?

CCing David, whose comment I was just transferring.

@Nikerabbit:

1. Who are the users who would use zh-Hant and zh-HK respectively?

This is bumped again by @A2093064 at Solution for different translation in zh-HK and zh-TW

2. Do we have translators willing to maintain zh-HK and understanding that overrides should only be done if the translations differ?

The translator section of Portal:Zh, shown that @Hello903hello @Justincheng12345 and @C933103 are actively translating messages to zh-hk, to distinguish significant different between zh-hk and zh-hant (which follows zh-tw grammars)

Change 431112 had a related patch set uploaded (by Nikerabbit; owner: Nikerabbit):
[translatewiki@master] Move zh-hk from no-export to always-export

https://gerrit.wikimedia.org/r/431112

Change 431112 merged by jenkins-bot:
[translatewiki@master] Move zh-hk from no-export to always-export

https://gerrit.wikimedia.org/r/431112

Is it possible to use language converter with specific conversion group on most messages and correct them only when language converter get it wrong?

It is not possible to use language converter on interface message translations.