Doxygen highlighting does not work in tooltip. The showed as simple text.
Yes, see changelog. This sometimes leads to crashes in clang. The feature needs to be fixed upstream before we can enabled it again.
As Sven said, this feature got disabled intentionally. See: https://phabricator.kde.org/D8857 for more information.
*** Bug 387748 has been marked as a duplicate of this bug. ***