Since the proposal aims at all such gates - what exactly makes (all of) them of historical interest to be put under the historic=* key?
- [They are] Not always suitable under
man_made=*due to cultural and historical significance
If a specific one is of historical significance, you may want to add historic=yes. But that’s not a blanket reason to avoid man_made. In fact, I think man_made=ceremonial_gate fits quite nicely, and is much closer to the current tagging which also uses man_made=*.
I also don’t really like the :type subkey. A better name for it might be shape to describe the outer appearance. Or just simply ceremonial_gate=* as the main value.