Proton Pass adds identity management:

Name

Email address

Phone number

Birth date

Gender

Address

Identity details (Social Security number, passport number, license number)

Personal details (personal website, social media profiles)

Work details (occupation, company, job title)

If you have a paid Proton Pass subscription, you’ll also have access to custom fields and custom sections. All fields in your vault are searchable, so all the information you need is safely stored and easily accessible at any time.

Paying customers could easily add a custom field here to store their Nostr private key. However, I feel there's a better way for all.