I know that it is already possible to have banner shields and to be able to put banners on your head if you have access to /hat or /head. It would be really cool if there was a way to put banners in your chest plate slot and wear it like a cape, this doesn't add much by doing this, but it would be a really cool addition to customize your character. Like I said, not much, but it would be cool if this was added. I don't even know if you can, there probably is a way, but if this ever does get chosen as something to add into PF, it would be amazing. It doesn't affect me a whole lot if this doesn't become a thing, I just wanted to share my idea and hear what you guys think about it. : )
Some great mods already exist for that purpose such as MinecraftCapes for forge or MoreCosmetics for fabric which are comlpletly free and let you add some very hd and animated capes (if u want).
I don't like or use forge, I know how to do all of that stuffs, I just did this for those who don't tbh
Any way to add caps w/o a modded client would require moving armor stands which would create considerable lag so this is not something we will be adding. Personally, I recommend using optifine!