The server can accept data from the client only from form elements?

Question

Backend

1. The server can accept data from the client only from form elements, and it turns out that the server needs to validate what the client sends?
2. And why on some sites the length of the password and email is limited, is this due to what?

Answer 1

forms are an outdated method, I defend it, it seems to me that
everything can be done through cross-domain ajax