I agree that "St. James' " or "St. James's" is a matter of debate. It
is considered better practice to omit the additional "s",

By whom?

I always thought that they were to show posession and to show missing
letters in words.
Anyone who watched 'Grumpy Old Men' a few weeks ago will know that Barons
Court doesn't belong to a Baron, yet Earl's Court belongs to an Earl.
Does St James'/St James's/St James/St. James Park belong to St James?
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