Governments that are script-rationed such as Basilike Patris, Saka Royal Land, or Imperial Seat are NOT lost forever when destroyed. They will be still lost though if cancelled while under construction. Tested it by playing as the Seleucids and destroying the BP in Seleukia while keeping the one in Antiochea. I had been reduced to between 13 and 26 settlements by then, so only 2 BPs can be built The next turn the BP was available in settlements like Sardeis.