[FaceType]

In sans-serif text typefaces capital ‘I’ is traditionally a simple vertical rectangle without serifs. But we were estimating some folks would miss those serifs so if you want to have a ‘properly-formed’ capital ‘I’ just activate stylistic set 02 (ss02, = ‘Alt I J i j r’). To see what this looks like, take a look here: https://www.monolisa.dev/tester/text

[MoonWalk]

Thanks for the reply, and that option. Pretty cool!

But the crossbars aren't serifs!

[FaceType]

You are correct. But the letter ‘I’ never has crossbars, no matter what font you are looking at. If you mean the horizontal stem, that is a serif.