Is the <form> tag mandatory?

Hi All,
Form submission earlier was done only using the “action”, “method- get or post” and button type “submit” within the <form> tags:
<form action=“login.asp" method="get”>
  <fieldset>
    <legend>Login</legend>
    <label> Username: <input type="text" name=“username”/></label>
    <label> Password: <input type="text" name=“password”/></label>
    <input type="submit" value=“Submit”/>
      </fieldset>
</form>

Forms submission are commonly handled using ajax these days. Submit button has a onClick function which handle the submission.
 <fieldset>
    <legend>Login</legend>
    <label> Username: <input type="text" name=“username”/></label>
    <label> Password: <input type="text" name=“password”/></label>
    <button  onclick=“Submit()”>Submit</button>
</fieldset>
So do we still be need to add the Form tag ? and does it help the screenreaders? Or <fieldset>  with proper labels and inputs elements suffice and are accessible in themselves.

Thank you,
Ashraf