You will be able to continue to use MongoDB in the v3 version, however once v4 is released we (the Strapi team) will no longer handle support for it and the MongoDB Team / Mongoose maintainer will provide “support”
With the release of v4 there will be no compatibility initially and we will be working with MongoDB / Mongoose to help them build a 3rd party connector in which they will handle support.
Our suggestion is that if you plan to migrate immediately to the Strapi v4 you migrate to a supported SQL database ASAP, we are still evaluating migration guides between MongoDB => SQL for now so we can’t provide much help there at the moment.