To allow dealers to offer the option of cheap and quick installation of the Datatool alarm/immobilizer fitment these plugs were standard fit on all bikes as they left the factory. In simple terms the plug completes various circuits essential to starting the bike.
When the Datatool alarm/immobilizer is installed it substitutes for that plug and when it is disarmed it replicates the internal bridges in that plug. However the standard plug should have been posted back to datatool by the installer as part of the warranty registration process, so in theory there should not be any of these plugs available in the wild - and certainly not on the Bay.
For your own safety you might wish to buy only through a third party account so that you are not telling the seller the precise location of a Street Triple which will quite probably no longer have an immobilizer installed in a week or two time so just may suddenly disappear. Sadly not everyone offering items on the Bay is wholly trustworthy. Meantime all vintages of Street Triples are still attractive targets for thieves especially if they know in advance they no longer have an alarm fitted.
In practice the parts people at most Triumph dealers will probably have one or two around simply to allow them to eliminate alarm system faults when fault finding. I suggest you ignore the Bay and instead call a dealer who already knows you and your Street Triple as they will probably be willing to supply a genuine replacement plug especially if you are able to show them the alarm you have removed.
Obviously removing the alarm/immobilizer from your Street Triple may need to be notified to your insurer.