As employers prepare T4’s, they can make like a bit easier for staff. By reporting the employees contributions to the health and dental premium (NOT life or LTD premium) in Box 85, you are helping staff make sure they get the medical Expense Tax Credit (METC) where available.
By reporting the employee share in Box 85 on T4’s, you avoid employees being audited for proof of contributions (and you having to write out letters for them).
You can find more about the history of box 85 and it’s intent here… https://www.canadianmoneysaver.ca/blog/box-85-is-my-box
The CRA info is below, or ask your accountant. https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/payroll/benefits-allowances/private-health-services-plan-premiums.html