Now LibreOffice export documents (forms) as PDF ver. 1.4. When Improved to PDF ver. 1.6.
See https://gerrit.libreoffice.org/53373 – actually LibreOffice 6.1 now defaults to 1.5; but just changing the version number does not change much. You have the effect that headers supporting only 1.4 give up early, but there is no magic that then all new features allowed by the newer markup version are also implemented. Best to file a bug at https://bugs.documentfoundation.org/enter_bug.cgi?product=LibreOffice if you’re interested in something specific.
In general, questions like “when will happen this or that” are useless here. They are like feature requests (which are off-topic here), but also with some demand to provide some estimation. That estimation is not available usually (unless it happens that someone already works on the feature); and usually the answer is “when some developer decides that it worth doing, has time and energy, and implements that”.
In LibreOffice development, individual developers work on what they are interested in; companies do what customers think important (or what interests the companies). No central planning or roadmap.
This is will be make if you find the developer, that realizes this functional for you