CoreText doesn't support GSUB8 at all?!

OK, this is going to cause me some serious issues.

$ cat foo.fea
languagesystem DFLT dflt;

feature liga {
    rsub A' B by C;
} liga;
$ python3 -m fontTools.feaLib -o foo.otf foo.fea test.otf
$ hb-shape foo.otf 'AB'
[C=0+1292|B=1+1327]
$ hb-shape --shaper=coretext foo.otf 'AB'
[A=0+1296|B=1+1327]
Mac OS 10.15.7.

Should I expect this to work? I feel like, since it's actually in the OpenType standard, it should really be supported, but...

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