TLDR - You should be able to purchase this illusion from the vendor using an original enchant scroll earned during this enchants time period, it would be very simple to implement this feature. It’s COMPLETELY ILLOGICAL that those who have rightfully earned this enchant still cannot transmog it.
I’ve seen countless threads about this issue, but since nobody has received any blue replies, I’ve decided to create my own.
The glorious tyranny enchant was available to those who received 2200 3v3 or 5v5 rating during the mists of pandaria expansion, there was also spirit of conquest and bloody dancing steel (I have all of the above). The issue is that to currently purchase this illusion, it requires duelist achievement earned in either season 14 or 15 (which was never the original requirement.)
If you have left over scrolls of these original enchants, you should be able to purchase this illusion from the vendor using these original scrolls as currency.
This would be such a quick and effortless implementation by the game devs and would really help out a lot of people that have this issue. Furthermore, if someone also has a weapon still enchanted with any of these from the time period as proof, they should be able to receive the illusion, on a case by case basis.
Edit : If you feel strongly enough about this issue (which I know most of us do), I would encourage you to tweet the devs @WarcraftDevs and let them know it! It seems like they rarely come to forums, as NONE of these topics have gotten any responses.