I always used to use a mechanical keyboard to type. Recently, I found a hardware trojan in my keyboard's firmware.
Can mechanical keyboard firmware be modified?
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I don't think this question can be answered directly without going through a list of specific keyboards.
However, even keyboards that don't have an updatable firmware could have their hardware hacked or their controller replaced completely by someone with physical access. For example, the Model M PS/2 keyboard probably can't be updated directly, but here is a project to replace its controller with one that can be updated.
It is near impossible to be secure if your physical environment can't be secured.