Downloading already didn't work for me since quite a while (the operation never completed) but what does this mean for highlighting files I write myself (or someone's mom does)?

Other users who do not follow the latest development releases [...] are likely not the target audience for downloading highlighting files.

I take a bit exception with this; users of certain distributions that aim to allow them to work *with* the software instead of *on* (as in applying updates) are being compared with a rather negative and probably sexist view of their mother. The opposite is true for many such users: working with stable releases means you have more reason to update things like highlighting files.

FWIW, there was never a reason to have a download/update button in the "userland" configure UI; a KCM provided by the highlighting framework would make perfect sense (that same KCM could serve to disable/uninstall useless highlighting files; your mom probably doesn't need more than half the ones that get installed.

Independent of mom/dad/somebody else, for highlighting files you create on your own, you just can perfectly add them to the local directories as specified in the manual.
If you really want to have new highlighting files and risk that stuff like indenters or included stuff might break, you can still get them from the syntax-highlighting git and put them in that folders manually, too.
The only thing that is no longer there is the "auto" download dialog.