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.

I use first@firstlast.tld I bought my firstlast.tld several years ago. Figure it would look good. I then put a modified resume on my domain but when I started to think about being a security professional that didn't seem like a good idea. I now have my domain bring up the IP, browser, and few other pieces of info and show it to whoever goes to the site. It is either that or blank page and I think the first is more fun.

I use my own first@last.com (well actually it's last-net.com because I couldn't secure any better domain. It's too common.)

Never had an issue.

It's all good, especially now that they have proton.me when it was protonmail.com I had some issues saying it over the phone as some people didn't understand and it is long to spell.

Shouldn't make a damn difference! Ask this person to explain their thought process

Proton leaked a user's IP address to law enforcement, you should seek alternatives. source: https://arstechnica.com/information-technology/2021/09/privacy-focused-protonmail-provided-a-users-ip-address-to-authorities/