Hi,
Refer to the code

Ensure that the manager has access to view all contacts, regardless of the custom filter. This can be done by setting appropriate access rights for the manager role.

Here is a sample Python code that illustrates how to implement such a custom filter in a model (e.g., res.partner for contacts):

from odoo import models, fields

class ResPartner(models.Model):
    _inherit = 'res.partner'

    visible_to_salesperson = fields.Boolean(string='Visible to Salesperson', default=lambda self: self.env.user.has_group('sales.group_sales_manager'))

    # Optionally, you can set access control for the custom field
    # visible_to_salesperson = fields.Boolean(string='Visible to Salesperson', compute='_compute_visible_to_salesperson', store=True)

    # Example of access control using a compute method
    # def _compute_visible_to_salesperson(self):
    #     for record in self:
    #         record.visible_to_salesperson = self.env.user.has_group('sales.group_sales_manager') or record.user_id == self.env.user

Hope it helps