Sic:
You can disable it in your alt-o window
Or UseNewUIEngine=0 in your eqclient.ini
Keep in mind, if you have issues where eq has a read issue to your file or can and will load with the "default" of being on - and if you're not locking your eqclient it can save that default value to your eqclient.ini
One of the reasons we (cwtn and I) added the new UI warning to our class plugins