We set the Sales Part to inactive in all sites (manually) and add a note for the reason of the discontinuation of the sales part - this stops adding the part to an order.  Other steps prior are taking in the quoting system to stop quoting some time before obsolescence.  You could set an even on the change of the sales part to Inactive that sent an email notification to an email group of next actions.  We don’t automate to that point, but could easily be done within the custom event and event action functions.  You would have to define the actions per site or only when inactivated on a certain site or whatever your intention would be to carry out the follow on actions.

Thanks ShawnBerk - you’ve confirmed my suspicions that this is the only sensible option