The new header HTML is riding the train and will be deployed as part of 1.29.0-wmf.13 which will roll out from the 21st February.
So as soon as that branch has been cut, you can remove any FIXMEs associated with T157987 and clearing the cache. They should relate to FIXMEs added in {{8180f4ab5e8f5fea6a0e7d6ad3da1db276b3149b}} and {{If1b014feac263ac5632cbbf797f597e43cf74868}} and are (but may not be limited to) the following:
resources/mobile.search/SearchOverlay.less 132:// Please remove entire block when the cache has cleared for T157987. resources/skins.minerva.base.styles/ui.less 123: // FIXME: form selector is only necessary until cache has cleared. Please remove promptly (T157987) 326: // FIXME: Remove when cache has cleared (T157987)
Please ensure that any styles relating to feature-header-v1 and feature-header-v2 are not removed at this time. These are captured in T156796.