First, a major change: after much discussion, Smogon is adjusting its usage threshold for Generation 8 to 4.52%, using unweighted months. In addition (and as was previously discussed in Policy Review), we are fully removing quickrises.
Why are we changing this? Well, last generation there were serious concerns with how disruptive tier shifts could be to lower tiers. While drops could always be handled via bans, rises could completely destabilize a tier, causing major impact... impact that then needed to be reset in a few months when the risen element fell again. Even a single popular sample team could end up causing major ripples in the tiers below due to nothing more than a passing trend. This was especially true if it occurred during the months weighted more heavily. While rises are always going to be a part of any usage-based tiering process, we wanted a solution that made them a bit less frequent.
So what does it mean to increase the tiering threshold? Well, the end result will be smaller tiers overall, where rises are less frequent but drops are more common. And ideally by going with an unweighted metric for tier shifts, eliminating quickrises and keeping quickdrops fairly low, we would also favor shifts based on consistent usage over a sustained period of time, rather than spikes in usage during certain periods. Also, a happy side effect of smaller tiers is that it allows lower tiers to be more practical despite the reduced Dex size.