If oracle supports this, then why does their documentation -- even the page you linked here -- claim that the trim character must be a single char -- the one example at this link is the only hint of multi-character support.
Even if we believe this example, it is ambiguous. They show that the characters 'OLD' are dropped from 'OLDMAN' if the trim_character is 'OLD'. That could be explained by 'OLD' matching exactly -- OR it could be explained by each character 'O', 'L', and 'D' matching any of the characters in the set 'OLD' -- so maybe TRIM('OLD' FROM 'DOLLMAN') is also 'MAN'?

On the other hand, mysql doc https://dev.mysql.com/doc/refman/5.0/en/string-functions.html#function_trim
unambiguously claims to support multiple-character exact matches

    mysql> SELECT TRIM(TRAILING 'xyz' FROM 'barxxyz');
        -> 'barx'

The remaining final "x" proves that it does not match any characters from the set in any order.

So, OK, our code can be mysql compatible. Maybe you should include the mysql link in the comment.