MMR Vaccine Does Not Contain Mercury, Thiomersal, Thimerosal and It Never Has
The specific information that MMR has never contained thimerosal/thiomersal is Table 1 and Table 3.
If UK readers would prefer a UK source then please consult MMR The facts.
Adding a preservative to MMR would be like adding a preservative to live yoghurt. Some people eat live yoghurt because they want the beneficial bacteria or even specially cultivated probiotics. If you were to pasteurise that yoghurt or add anti-bacterial or anti-fungal preservatives, then you would inactivate those potentially beneficial bacteria so you wouldn't get the benefit. It's a similar story for live vaccines such as MMR and thiomersal/thimerosal.
Kevin Leitch Canards Project is an opportunity to put together a resource that answers FAQ or common concerns; please contribute questions or answers to it.
Measles Is a Trivial Illness and There's No Point to Vaccination: The Myth