Select Fields should support disabled option out of the box

While implementing multiple log-in systems I noticed that there's no way to implement disabled select options without significant effort on routes and templating.

Would a solution inside flask-wtf be appreciated if I make a PR and what would be the desired approach?

Below the solution I took.

Frontend

Required Code

# forms.py
# Admin Log-in form for admin interface
class AdminLoginForm(FlaskForm):
    admin_login_username = StringField('Admin User', validators=[DataRequired()])
    admin_login_password = PasswordField('Admin Password', validators=[DataRequired()])
    admin_login_rememberme = BooleanField('Remember Me')
    # Currently only PAM, for future use when we add LDAP/AD support can be enabled to selectable realms
    admin_login_realm_options = [
        ("pam", "Linux PAM Standard Authentication"),
        ("ldap", "LDAP"),
        ("ad", "Active Directory"),
    ]
    admin_login_realms_enabled = {"pam"}  # only PAM is allowed for now, but this can be easily extended in the future

    admin_login_realm = SelectField(
        "Authentication Realms",
        choices=admin_login_realm_options,
        validators=[DataRequired()]
    )

    admin_login_submit = SubmitField('Sign In')

    def validate_admin_realm(self, field):
        if field.data not in self.admin_login_realms_enabled:
            raise ValidationError("Selected authentication method is not enabled.")

# routes.py
@ssldeploy.route('/admin/', methods=['GET', 'POST'])
def admin():
    brand = {'brandname' : 'companyname'}
    form = AdminLoginForm()
    if form.validate_on_submit():
        flash('Login requested for user {}, remember_me={}'.format(
            form.admin_login_username.data, form.admin_login_rememberme.data))
        return redirect('/')
    return render_template('admin/adminlogin.html', title='SSL Deploy Admin Interface', brand=brand, form=form)

<!-- Select Form Field Start -->
                    <select id="{{ form.admin_login_realm.id }}" name="{{ form.admin_login_realm.name }}" class="loginformfieldrealm">
                    {% for value, label in form.admin_login_realm.choices %}
                    {% set disabled = value not in form.admin_login_realms_enabled %}
                        <option value="{{ value }}"
                        {% if form.admin_login_realm.data == value %}selected{% endif %}
                        {% if disabled %}disabled class="text-gray-400"{% endif %}>
                        {{ label }}{% if disabled %} (disabled){% endif %}
                        </option>
                    {% endfor %}
                    </select>
<!-- Select Form Field END -->

Start by reviewing SelectField and how its choices are rendered into the HTML select and option elements. Compare that path with the forms.py, routes.py, and template example in the issue. Done should provide a supported way to mark selected options disabled out of the box while retaining appropriate server-side validation.