In T290936, DiscussionTools needed to know about any fostering on a page to avoid corruption.
Several iterations of patches tried to expose lints of fostered rendering transparent tags in a way that wouldn't be too disruptive of editors work (only lint when not template generated and outside the template namespace),
https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/+/721355
https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/+/1052192
but it ultimately proved too disruptive and was reverted,
https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/+/1054407
A new approach (T369317#9983813) would be to introduce a new hidden lint category, "fostered-transparent", that reports these lints separately from "fostered".