When implementing the banner for IP masked users in T330510 we intentionally limited the design to the Vector 2022 skin and Minerva skin, as implementation was technically challenging for the other skins.
Given Wikimedia sites do not present Timeless, Monobook, Vector, CologneBlue, Vector([1] or Modern to anonymous users and we lacked clear designs for these skins (which use absolute positioning) we descoped this for now.
3rd party sites that use these skins and support anonymous editing would potentially need updates to these skins (and mocks).
Developer notes
Skins can enable the banner by adding "tempUserBanner": true so the options in their skin definition like this example in https://gerrit.wikimedia.org/r/c/mediawiki/skins/Vector/+/930849
For skins which do not rely on absolute positioning incorporating the banner will be easy, but skins that do use absolute positioning will need some changes to support IP masking. This banner by default will not display correctly in the skin so will require changes in the skin to be incorporated.
For example here is how the banner looks in Vector currently without changes:
[1] We recognize several sites have Vector as the default skin, but we are not clear on the timeline for IP masking/desktop improvements and whether this needs to be fixed beforehand.