It seems this issue have been fixed?

"disable" originally means not showing the varinat in the variant menu. However, the functionality has been removed a while ago.

The question is why this translation update is needed.

More specifically, this is a clean up of the duplicated translations, as zh-tw namespace names, namespace aliases, special page name, special page aliases in .php files should be stored in zh-hant language files as how zh-tw/zh-hant .json translation files do.

languages/messages/MessagesZh.php

• line 49, 54, 55, 59, 60, 61, 67, 68, 69, 70, 96, 97, 98, 99, 104, 105, 106, 107: adding missing namespace aliases from zh-hant and zh-tw namespace aliases as all variants' namespace name translations should be added as zh namespace aliases.
• line 121, 123, 132, 164, 172, 177, 196, 198, 200: adding missing special page' aliases as all variants' special page name translations should be added as zh special page aliases.

By who is this wanted, or why is this needed and which problem does this solve? This task describes a solution, but no problem to solve.

The syntax here should meet what you need to keep the old namespace names as aliases:
https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Scribunto/+/702950

The syntax looks weird to me.

This commit will directly fix the issue https://phabricator.wikimedia.org/T165593 in extension.

Also, if possible, please update variantname-zh-tw in languages/i18n/zh.json to "臺灣正體" according to the discussion: https://zh.wikipedia.org/w/index.php?oldid=53225398#「台灣」「正體」？

So only variantname-* messages should being kept in those json files.

https://gerrit.wikimedia.org/r/c/mediawiki/core/+/696351/1/languages/i18n/gan.json

I'm not sure how the variant translations are done for zh.

All variantname-* messages should be kept in the base variant/language file to prevent being fallback to specific variant files.

https://gerrit.wikimedia.org/r/c/mediawiki/core/+/696351/1/languages/i18n/zh.json

https://zh.wikipedia.org/wiki/Wikipedia:互助客栈/技术?oldid=66124050#為甚麼選擇地區詞那邊的「繁體」選項都變了簡體字呢？

The variant label (variantname-* messages) are supposed not being changed when changing the current display variant (?variant=).

Namespace name in page title variants are also affected and only display en namespace name.

See T230681 .

The Translated title in https://www.mediawiki.org/wiki/Download/zh is "下載", when changing the display variant into zh-hans ( https://www.mediawiki.org/wiki/Download/zh?variant=zh-hans ), the title should be "下载", but it didn't convert "下載" into "下载,"