Is there any reason we don't allow full URLs for bug ids? i.e. https://phabricator.wikimedia.org/T321134 rather than just T321134?
Topic on Talk:Gerrit/Commit message guidelines
The Gerrit (and now GitLab) integrations with Phabricator do not expect or support full URLs. We could look into changing that, but I would say that's where the convention comes from.
There is I believe a long standing tradition in software development to refer to tickets and issues by ID.
Eg. "Fixes #123" in Trac, "FOO-123" in Jira, "bug 123" in Bugzilla, "D123" at Facebook/Diffussion, "Issue 123" in Monorail/Chromium/Google. And more recently on GitHub with "#123" and "gh-123" to mention, resolve, or close issues and pull requests. Phabricator does this as well between tasks.
In MediaWiki, we adopted this during the time we used Bugzilla. You also see it in code to refer to tasks more succinctly. I suppose we value visual brevity, and it does seem appealing to explain (and support/maintain) only one way to write/parse something.