In general, at the time of declaring a custom event, you can specify whether the skript should process this event only synchronously.
ex:

custom event "evt"
   pattern: "my_evt"
   sync-only: true
  #sync only: true (second pattern)

Initially, now, any custom event can be processed asynchronously (for backwards compatability), unless 'sync only: true' is specified
Why not allow async, I thought that since initially any event is now processed asynchronously (if possible), then sync only sounds more logical.