Promise not to remove Sentinel?

[maartenbreddels]

In bbbdfbb 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:

• DeprecationWarning: Sentinel is not a public part of the traitlets API. It was published by mistake, and may be removed in the future. jupyter-widgets/traittypes#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.