SPL won't gonna happen! There is LL that is quite good and they started to add support for it in the CubeMX. The problem is, when LL will become a threat to HAL, they will stop adding support for it. I wrote an application in HAL that had 8K code size and it downsized to 2K when I ported that to LL
https://nucleobytes.blogspot.ro/2017/09/alea-iacta-est.html so for me, here is where my adventure in HAL ends for good.
At 4.23.0 version, CubeMX still offers a "Makefile" project generation, independent of any IDE, that will easily allow you to add your own libraries. But at this version they broke it intentionally, adding garbage in the Makefile to scare the beginners
https://community.st.com/thread/43473-undefined-reference-to-llrccgetsystemclocksfreq . I still continued to develop applications and libraries this way, archived them and posted in that thread, but after a weekend of moderation, the forum got "crashed" Monday. When it went back, my post was gone. So I'm pretty sure that they don't want this way of doing things to be spread among beginners as it will mean pretty much the end of HAL.
So they will continue pushing HAL as they recently acquired TrueSTUDIO and it will be offered for free in the professional form. Of course, I still have to wait to see the next version of CubeMX and the way things are progressing to make any statements, but this is what I experienced with them 'till now.