Proton Mail contacts → Android system address book.
A GPL-3.0 Android app that signs in to a Proton Mail account, decrypts the user's contacts client-side, and exposes them to ContactsContract so they appear in the system Contacts app (and any other app that reads contacts) the same way WhatsApp or Telegram surface their own contact directories.
I wouldn’t use a package that does that because it breaks the security of proton, but tbh it’s email who cares.
As far as that package in particular who can say.
From a more 10,000 foot perspective: if you have to ask on a public forum if an application you give access to your secure email account so it can copy and paste contacts into your os address book is safe it’s probably worthwhile to figure out what you’re trying to do and under what constraints.