The ability Mirror Shield grants triggers and resolves before combat damage is dealt. That creature won’t deal combat damage to the equipped creature or be dealt damage by it.

If a creature with deathtouch blocks the equipped creature and is then destroyed, the equipped creature remains blocked. Unless the equipped creature has trample, it won’t deal combat damage to the player or planeswalker it’s attacking.

Once a creature without deathtouch has blocked or become blocked by the equipped creature, causing the other creature to gain deathtouch won’t cause the ability granted by Mirror Shield to trigger.