3. How (if at all) has the percentage of logged out traffic we can include in a controlled experiment increased from the current 0.1% threshold?

To clarify, the 0.1% threshold is set specifically for English Wikipedia. The other wikis have a 10% max traffic threshold. These limits are set for any experiment that uses the edge_unique identifer, which is needed if we want to include logged-out users in the experiment.

See current Test Kitchen documentation and related task to investigate impacts of increasing the enrollment sampling thresholds.

Test Kitchen guidance also currently states

If you need to have trustworthy insights about the effect of your change on logged-in user behavior, we recommend conducting a separate experiment (either in parallel or as a follow-up) specifically on logged-in users, using mw-user as the identifier type.

This will be important to consider in our experiment design if our primary metric is focused on the impact to registered users.