It was deprecated in https://gitlab.gnome.org/GNOME/libxml2/-/commit/75b5bc2b0a6219ee03069611793d..., with no MR or other explanation. Maybe the expected replacement is setting xmlTreeIndentString in every thread?
We could do that, we have a DllMain right there, but that'd initialize libxml state even for unrelated threads. Feels like there should be a better solution somewhere... but I don't know where.