If we’re continuing to do standalone WiredTiger releases, we should consider incrementing the Btree minor version. It’s not needed for MongoDB, but it’s likely needed for other applications.
Incrementing the Btree minor version would require changes to the mongodb-4.2 branch to support the new minor version (otherwise we'll break MongoDB downgrades), so this decision should be made before we push the 4.2 update allowing that release to handle the 4.4 written on-page cells.
Changing the Btree minor version number would need strong wording in the upgrade document, as it would be impossible to downgrade after running under the new WiredTiger release. Alternatively, we could could backport changes similar to the ones in the mongodb-4.2 branch, to a previous WiredTiger release, so the older release can read (and ignore) the changes in the mongodb-4.4 cells, the same way 4.2 does.