Hello Debian LTS team! I think that is *if* makes sense to engineer some automatic upgrade path in an LTS release, then it would be to introduce MariaDB 10.1 into Jessie. Upgrading from MySQL 5.5 and MariaDB 10.0 to MariaDB 10.1 is pretty safe, and the maintenance period of MariaDB 10.1 would match the Debian LTS needs. MariaDB 10.1 is very stable, and already maintained in Stretch anyway. MariaDB 10.1 also has a good track record of being used for a massive MySQL 5.5 -> and MariaDB 10.0 -> upgrades, so it definitely is the least risky of the upgrade options now and in the long term.
However, if such a feat is attempted, there should be extensive (and automatic) testing first. In order to prepare that I filed a MR on the Salsa-CI system that jessie would be a supported target for Salsa-CI in the first place.. https://salsa.debian.org/salsa-ci-team/images/merge_requests/26 If any of the Debian LTS members have write permissions to the Salsa-CI-team repos, please merge :)