According to this bugreport on Bugzilla, this feature was removed somewhere around october 2018 due to security concerns:
We explicitly decided to disallow deep linking into internal settings
to reduce attack surface, present and potential.
To make a current example, "Open in New Tab" on an "about:config" link
on the web works after a reload, even though the first load is
disallowed. This probably wasn't foreseen originally, and we may have
similar cases in the future.
Less steps for users isn't always better, see for example how the
security certificate override dialogs are explicitly designed to slow
down such operations. Deep linking into "about:config" to instruct
users, for example, to disable those security checks in less steps
could make it more attractive for attackers, and the link availability
may seem like an official endorsement of the use case.
Since the report is marked as WONTFIX
, the feature most likely won't be reintroduced in the future.