It would be helpful for WiredTiger if we could turn on some assertions about timestamps, and as discussed in a recent meeting we should try to enable the mixed_mode timestamp assertion. I have attempted this and created a MongoDB patch build to check the fallout and there was still a significant amount. This is a shared ticket and will require a developer from the WiredTiger team and SERVER as fixing the test failures will be an iterative process.
When encountering a test failure a separate ticket should be created to fix the failure (if possible) and then this ticket will proceed once that ticket is completed, with the goal of one day enabling the assertion and merging it to master.
The patch I used to enable it was (it may not apply correctly so just take the config change):