You are right, not finding any documentation on OPTIMIZED / UNOPTIMIZED. The syntax change seems to have been made for 18.2. Here are the set / show value in that version:
And you actually cannot set ON and OPTIMIZED in the same SET command. Weird! Maybe it is something that is planned but has not been completely implemented?
it's a bug...optimize is pretty much useless
So I told Jeff it was useless earlier but, maybe not so much. We've logged a bug to fix the message as its misleading and add doc for the "feature"
Serveroutput is presented normally after each statement so its presented in line at the right place. However, the _only_ time thats really relevant is when you have a dbms_outut in a trigger or something. (imagine worst case of that in an insert trigger)
The optimize flag passes over the in script get serveroutput and presents it at the end instead which stops lots of calls to get serveroutput that for the most part are not useful.
This immediate issue will be fixed in next release and the doc updated.
Thanks for explanation!