Clarification on `//link/must_be_base_link` element

While addressing a downstream issue https://github.com/robotology/gym-ignition/issues/385, I discovered the existence of [`//link/must_be_base_link`](http://sdformat.org/spec?ver=1.8&elem=link#link_must_be_base_link). After [checking upstream](https://github.com/ignitionrobotics/sdformat/search?q=must_be_base_link), I only found references to the specification, but I didn't find any place where this element is actually read.

Is it used elsewhere? Should it be deprecated in favor of the new [`//model/@canonical_link`](http://sdformat.org/tutorials?tut=pose_frame_semantics_proposal#2-1-implicit-frame-defined-by-model-pose-attached-to-canonical-link) attribute?

## Environment

Issue related to SDF specification.

## Description

* Expected behavior: clarify if this element is used and read at least in Gazebo (Classic / Ignition)
* Actual behavior: this element seems ignored

Start with the SDF 1.8 specification and the upstream search for `must_be_base_link` linked in the issue, then check whether Gazebo Classic or Ignition reads the element. Done means documenting whether it is consumed and resolving whether it should be deprecated in favor of `model/@canonical_link`.

Written by the indexing model from the issue text.