I did some research to know which kind of armor would be able on that setting. I concluded that heavy armor is extremely rare, because in that time they rarely cover all their bodies with armor, they rely more on shields and cuirass.
So heavy armor (taking in account cataphracts) will be limited to chain mail and splint mail. Medium armor is the way to go to a "decently armored" soldier, and light armor is given to the more expendable or less advanced infantry.
So this forces PC's to rely on DEX to achieve good AC's, probably a 14. At the end that would mean a difference of 1 AC (half plate VS full plate).
In this scenario, ring mail and plate are banned; chain mail and splint mail are escarce. Half plate will resemble something like the roman lorica segmentata.
At this points, everything seem almost fine, DEX becomes a bad dumpstat, that's ok, more stealth around. But there is more factors to consider for weapon banning.
That time weapons were made to match their enemies armor, and weapon technology try to counter the defenses or new threats.
Because there will be dragons and some other thoug evil meanies on that setting, some weapons would still makes senses, even if they don't fit historically, for example the greatsword, you have a sword, which is larger and cur better. If I don't recall wrong, greatswords were made to counter heavy armor, so It makes senses to me that they made greatswords to deal with dragons necks.
Greatswords don't need greater technology or theorycrafting to make them. Not like, for example, a morningstar or a flail, specifically made to overcome full-plate.
After some thinking I ended up banning: halberds, flails, morningstarts, rapiers and crossbows.
Did I miss something or is a reasonable setting implementation?
Thank for your answers, and sorry for my english, it isn't my native language.