IMHO it is not part of the frame, it looks like it was welded on and painted over.
Is your bike 2nd hand or was it bought new ?
It could well be it was an add-on by a previous owner for whatever his requirement.
However, we only have a pic from one side of the tab.
Take a pic from the other side as well, enlarge, and see what the tab is for.
If "it's just there", then you have a case for getting rid of it.
If it "does stuff" then it has a reason to stay.