Thank you for this font!
I suggest you to draw pipe character (|) always vertical, even in italic mode. Because it could be mixed up with slash (/) characted. Many programming fonts do this, for example Hasklig. Also look to screens.
FiraCode:

Hasklig:

Hasklig is based on [Source Code Pro], that has an italic variant, while Fira Code's base ([Fira Mono]) does not. So it's actually your editor/text rendering engine that makes pipe symbol appear slanted.
I'm afraid it's not something that could be easily fixed within font itself.
@sainaen is right, I just want to add that this sort of thing _should_ be fixed in the font, by providing the proper italics. Unfortunately, it鈥檚 like developing the whole new font: it鈥檚 out of my reach.
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Hasklig is based on [Source Code Pro], that has an italic variant, while Fira Code's base ([Fira Mono]) does not. So it's actually your editor/text rendering engine that makes pipe symbol appear slanted.
I'm afraid it's not something that could be easily fixed within font itself.