Did you ever issue ROFR (Right of First Refusal) for angel investors? Can it be a red flag for VC on demo day?

It would be odd. You can offer pro-rata to an angel if they asked, but not Right of First Refusal.

It would be a big potential red flag. It would mean that the angel would have the ability to block any future investors. VCs will definitely have a problem with that. Check with your lawyers before signing any ROFRs - even for VCs.