I seem to recall it being frowned upon except in the case of Jesus, who traditionally takes an unaccompanied apostrophe.
I vaguely remember being taught something similar - Jesus and ancient Greeks (both real and mythological), iirc.
I expect it's one of those rules like not splitting an infinitive that's based on something from classical education and is best ignored in the 21st century.
Apostrophe-no-S (eg Bridget Jones' Diary) is house style for the publication I work on. I don't like it but house style is more about consistency than correctness. Other publications have apostrophe-S in their style guide (Bridget Jones started in the Independent, iirc, so if the "correct" form is Bridget Jones's Diary, all that means is that their house style is apostrophe-S).
My job description includes guardianship of the style guide and I could, in theory, change the rules if I wished. However, they predate me and it would be difficult to implement such changes for reasons that are too tedious to explain here, so we're stuck with apostrophe-no-S.
d.