https://zwiftinsider.com/anti-sandbagging-plans/
Controls for sandbagging starting to be introduced to Zwift (code already in the current release, but will be rolled out to specific events to begin with and then scaled up).
Zwift will warn you when you are attempting to join a category for which your 1min and 5min power are too high (but will still let you join). If you break an unpublished limit for the 1min and 5min power intervals during a race your pace will be limited and a logo will be applied to your character to indicate that your pace has been limited.
If event organisers get the option to turn on the "prevent sandbagging" feature so people with 1min and 5min power numbers higher than the category they're entering then it will solve most of the problems.