GitHub itself does not linkify text like "ICU-23456" to point to the Jira ticket. You can get links via browser extensions.
Install the Jira HotLinker from the Chrome Web Store.
Configuration Options:
Jira instance url: https://unicode-org.atlassian.net/
Locations: https://github.com/
https://github.com/unicode-org/icu-jira-safari
Install from the Mozilla Firefox Add-ons site:
https://addons.mozilla.org/en-US/firefox/addon/github-jira-issue-linkifier/
Source:
https://github.com/jefgen/github-jira-linkifier-webextension