There's actually a section in the docs around deploying Tyk to production:
With regards to MongoDB - we would always recommend setting up a resilient MongoDB replica set. However, if Mongo goes down, the only issues you will have are that analytics will not be written and that data will be lost.
Also, if mongoDB goes down, the updates to the definitions stored in Ttk will fail - so if you hot reload a Tyk node cluster with a downed MDB, then they will load empty data and those Tyk nodes will no longer proxy traffic for your APIs, but this is a very edge case, since most hot reloads are performed by the dashboard, which will not work correctly if Mongo is down.
So, in conclusion, MongoDB is not mission critical, but we still recommend having a resilient setup.
In the next version of Tyk, the MDB dependency for the nodes is completely removed and analytics purging services have moved to a new component called Tyk Pump. This also means the failure edge case I mentioned above can't happen, since we've made that more resilient too.
That means the MDB is only a dependency for the dashboard/portal.
Hope that answers things.