In T147696 (run in 2016) we saw that cologne blue had 45 very active editors, 324 active users and 2 544 editors with at least one edit.
For nonactive users there is no indication that this skin is badly supported (see T178422) and adoption is not encouraged.
We have discussed deprecating this skin for many years but we have done nothing about it. In the meantime we cause embarrassment to ourselves by irresponsibly presenting it as an option despite the fact we barely maintain it and there are many obvious visual bugs. At the same time we invest time making things work to avoid seeing fatals in production at the cost of slower development in the skins we do maintain.
As an experiment we will see how hiding cologne blue in preferences impacts usage and can in future use this as an interim step to deprecate skins if deemed necessary.
Practically we will add cologneblue to wgSkipSkins.
This will allow users to continue to use this skin but will hide the skin option for those that have not opted into the experience. For users who opt out of the skin and then want to opt back in we hope we can use this to start a narrative about the cost of such support. These users will also be able to opt back in by adding useskin=cologneblue in the url of the preferences page
We can of course at any time reverse this decision. It's a config flag but hopefully we can at least identify people that need this skin and work with them to realise their needs in other more sustainable ways.