Well, first thing I'd do is make sure it's not already researched and you just didn't realize. Check the "CRAFTING" section of the build menu for the Heavy Armour Smithy and Plate Beating Station.

If it's not there, what I'd do is start disabling mods (backup the mods.cfg file to back up your mod list and load order). Could be one of them is either moving the building in the list, or altering the tech.

looking at my mods theres a building mod called forgotten buildings? maybe that could be it

yeah nah still cant find it

What I'd do to test mods en masse is back up that file I mentioned, then just disable all of them. Don't have to save after loading, just go in and check if the building/research is there. If it is, you know it's a mod and you can start doing more precise tests. If it's not, you know it's not a mod problem and you can stop looking there.

Make sure to look for arrows on the build menu.

If it's missing in the tech tree and you're running mods - try teching into quarry and copper alloys.

Mods like Genesis, use advanced resources for advanced armour and have them as pre-requisite research so that one can't end up with the bench but no materials unlocked.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2123174859
examples of arrows sometimes missed by players

Found it btw