Protonmail is a widely used and common email provider. There is no reason why an employer would be prejudiced against your application based on you having a Protonmail address. I think a far more common thing employers think about when seeing applicants’ email addresses are things like “haha, they’re still using their email address from when they were 8 of alexdaboss at gmail dot com”, but I highly doubt they care about what domain it’s on unless you’ve got like a pornhub.com address or something.