@jareddavison I suspect that the limitation is related only to Oblique font slant, not Italic. I am now without my mac to test it. Can you test it?
You can edit the .fmx file directly in the notepad. The follow line is for FontSlant=Italic:

Font.StyleExt = {02040000000100000004000000}

The follow line is for FontSlant=Oblique:

Font.StyleExt = {02040000000200000004000000}

Off-topic: Just to who it may interest, I also made a tool to convert font weight, slant and stretch to TFontStyleExt and vice-versa as you cannot choose it using the IDE UI on the previous versions of RAD Studio:
Font.StyleExt.zip

Thank you @viniciusfbb . that's interesting but I don't really understand the implication. Is this a style mapping issue?

See the design time (Windows) and runtime (MacOS) versions of the form:

Updated project files:
SkiaItalicsTest2.zip

For interest's sake I thought I'd also show there is a slight difference between the design time and runtime on windows. I suspect this is because design time is not using SKIA where runtime is, am I right?

@jareddavison I don't know exactly the reason for this yet, and I really want to investigate more about this first. However, now we already have a possible workaround:

  SkFontSlant : array[TFontSlant] of TSkFontSlant = (TSkFontSlant.Upright, TSkFontSlant.Italic, {$IFDEF MACOS}TSkFontSlant.Italic{$ELSE}TSkFontSlant.Oblique{$ENDIF});

I suspect this is because design time is not using SKIA where runtime is, am I right?

Yes. Design-time does not use skia because the GlobalUseSkia is False by default and your application change it only in runtime.

Here's the results of the patch on MacOS.

Also, here's the results for iPhone simulator:

@jareddavison The user Hans Jakobsen said on RSS-1231 that Italic Bold does not work. Can you confirm if that workaround works for italic bold also?

@viniciusfbb, I couldn't reproduce it as reported by Hans Jakobsen. See image below. Using the unpatched FMX.Skia.Canvas, bold does appear bold, but Italics was broken.

@jareddavison Thank you very much!