To expand and give some notion of good taste:

It's more about writing

    struct User {phone: MaybePhoneNumber} // give or take, it's a monoid

    struct User {phone: Option<String>}

I don't mind discussing syntax when appropriate, but this feels like arguing over which trivial brainfuck substitution[1] is the best.

> monoid

nullables with `??` and `?.` are also give-or-take monoids. is it common though to `or` two MaybePhoneNumbers together or to apply a PhoneNumber->MaybePhoneNumber function to it? if not then why mention it?

let's see something meaningfully different like a database schema.

[1] https://esolangs.org/wiki/Trivial_brainfuck_substitution