Promise not to remove Sentinel?

In https://github.com/ipython/traitlets/commit/bbbdfbbf57d06487e3acd54d954fe512e5d1b693 Sentinel was made deprecated. In the warning it mentions it might be removed `"It was published by mistake, and may be removed in the future."`.

Sentinel is already used in traittypes, and now people are afraid this will cause a breakage in the future:
* https://github.com/jupyter-widgets/traittypes/issues/47

I think there is a tendency in the Python community to remove deprecated 'things', I think we should not do that.
Instead, we should try not to touch the deprecated parts, especially if they are 'islands of code' like the Sentinel class is, since it would not increase the maintainance burden at all I think.

Removal and renaming have caused a lot of pain and suffering for library maintainers and users and caused a lot of trust issues.

I suggest we either promise to not remove it, or mention we try to not do that.

Start by reviewing commit bbbdfbbf57d06487e3acd54d954fe512e5d1b693 and the linked traittypes issue to understand Sentinel's deprecation and downstream concerns. Done would require a decided project policy on whether Sentinel remains supported, with that decision communicated in the appropriate project documentation or deprecation guidance.

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