Hard to take back an already open can of tuna from the cats; you know, cat (and developers) can be very protective of what they have already gotten.
Out of curiosity, are the external tables filtering particular ORA- events?
Answering your direct question, I do not think Oracle has a particular best practice recommendation about not allowing developers to see it. I know, I know... a bunch of non-DBA looking at the alertlog can be an annoyance, I feel for you.
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