There's a MongoDB binary downgrade contract with the following requirements (among other caveats):

1. FCV must be set to the previous version
2. MongoDB must perform a clean shutdown

Right now, MongoDB validates that FCV is set appropriately, however, the clean shutdown part is only enforced by WT. The two ways WT can enforce this are:

1. Bumping the log version
2. Advancing the release compatibility (with coordination of changing MongoDB compatibility=(release,min,max) calls into WT)

There's a (testing) cost associated with branching a new development version of MongoDB, but delaying the bumping of WT versioning (via either mechanism):

1. "implicit" multiversion tests can perform illegal downgrades (typically via crashing the process, or having some restarts in standalone mode). But that illegal downgrade won't be noticed until a formal versioning change is made, which results in a bunch of fallout at once that lands on the wrong team.
2. explicit multiversion tests can depend on MongoDB startup failing. Right now, a "bug" where startup works after an illegal downgrade results in a test hang.