Is it really not possible to show a custom field (not stored value) on an email template?

I'm pretty sure not because that value isn't stored in the DB, it's just calc'ed every time you bring up the form that contains the field. It's clunky, but I've gotten around this in the past using a workflow. Set the field (or probably a new field) to Store Value (so it will then be available to select on an email template), then create a workflow that does the same math that the formula field was doing.

Thanks. I just wanted to confirm since I am able to use a non-stored value in advanced pdf templates. Hoping the same would be the case for email templates.

No it’s impossible

I am able to use a non-stored value in advanced pdf templates

I didn't know that was now possible. That's a new development. Sweet!

Yeah. It saved me from creating duplicate fields just to store value via a workflow like you said. Hope NS can do that as well for email templates soon