The recommendation is to always use properly escaped URLs, especially if they are hardcoded. The characters [ and ] need to be escaped as %5b and %5d
When URL that hasn't been properly escaped is used, the automatic escaping system is used that in some cases is not capable of escaping as URI syntax is ambiguous and by having only URI it's not even possible to know what the result should be. We don't want to introduce regressions by adding more escaping cases, hence we don't intend to fix this for Unity 6 and older.