Those could had been implemented loooooooong ago. In a hacky way. The dictate was to be pure and wait for GEGL. So still nothing of that. We got layer trees at least.
They could have been implemented by overloading the layer blending code, and recoded with GEGL once ready. But pure won over "we can do that too now, please do not look behind the curtain". It's really simple: adjustment is just a layer that only uses mask data and operates on the bottom layer pixels with already coded functions (levels, contrast, etc). Styles would be similar, applying plugins/scripts. If when proposed the reply is "we will not accept that kind of crude system", nobody wastes time coding a patch.
That was not the only time GIMP project went the pure road. There was also drama with themes and paintbrush system. On the end, some of the old ideas managed to get there, like grey icon theme (now hated by some, the irony) or having a working paint system (I have not tested with a tablet, so no idea if any good). Even non coders were trying to help, for example contributing brushes and testing the system [0], yet they ran out of steam after some time of hitting walls. The few people involved now seem to be the ones that have been taking over after all the previous got seriously burnt (except Kolås).
[0] https://www.ramonmiranda.com/gimp-paint-studio-su-pasado-pre... (autotranslation work OKish)