Undo deprecation warning - should be regular warning?

[lbdreyer]

Whilst looking for deprecations that needed removing in the Iris 3 release, I came across what looks like something that shouldn't be deprecated?

I'm not sure! So I thought I would raise it.

It was originally a regular warning, but #1993 changed it to a deprecation warning

[bjlittle]

@pp-mo Unfortunately, your name is against #1993... care to cast your mind back to 2016 and comment, if humanly possible?

[pp-mo]

Nice spot @lbdreyer !
I think this is a plain error from the original PR #1993.
Probably from an over-zealous use of search+replace.

I don't think this has anything to do with a deprecation.
It's also a pretty useless message, as-is, as it doesn't provide any context for what is wrong and only makes sense in reference to the API of a private internal routine.
:missing emoji for cupping brow with closed eyes: