Honestly, does it matter? The notation is in use, both on normal-sized and grace notes. Let’s leave the interpretation to the players. Maybe, if the notation is unclear, they will need to consult the composer, or a musicologist who decides he/she knows it, but that’s not the question here.
In the Brouwer example, there are even three slashes. Even more faster, I think. I’ve never seen it before.
Except that Dorico does not permit slashed notation on normal notes (as far as I’m aware). Normal notes occupy normal time. Slashed noted do not.
That, of course, is also a question to be asked even of the humble single grace note: does it “borrow” from the next note’s duration of the previous note’s? Practice varies. (I’d argue that in, for example, the late 18th century it often serves more as a sort of proto-Schenkerian analytical symbol, showing the harmonically subordinate status of the grace note even though it will be played on the beat and cut the duration of the following note in half.)
Of course, this doesn’t negate your point about the grace note(s) of yore being wedged between other notess unfolding in measured time (except when approaching a fermata), Janus. (Which, incidentally, means that even grace notes “occupy normal time” in a more general sense.) But I think it does show a bit of conceptual nebulousness regarding their precise temporal meaning — whether slashed or not — even before the more recent “flurry-of-notes” use of the symbol.
I have an un-researched theory that for composers like Brouwer, the triple-slash notation signifies a sort of “unfolding” of a complex (>2 notes, ordered) tremolo. If so, it takes them out of the category of grace notes completely, and the burden is on the boxes to designate time.
Yep - that is well understood.
The question here seems to be a bar with a bunch of slashed notes and not a lot else besides to give context.
Personally I think it is just sloppy notation, but I fully expect the modernists to ridicule me for that sentiment.
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That’s ridiculous: we’re nice, friendly folk who only want what’s best for performers and conductors alike… 
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If they’re supposed to be as fast as possible, or rhythmically free … that is what grace notes mean. I still don’t see any reason not to write grace notes and scale them as needed. They even have the advantage of automatically not spacing according to the prevailing rhythm.
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This is exactly why it’d be great to be able to add a slash to regular notes!
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The spacing option would be useful also. Horses for courses as they say …