Hmm ... my experience is that the notification on adding items to your cart expires, but when you place an order it will get cancelled several hours later with the following message (E6). Reading it back I do see that they say that limits are reviewed and changed regularly, so my earlier statement about it being "for LIFE" is indeed not correct:
Dear xxxxx,
We’re really sorry, but your recent LEGO® Shop order Txxxxxxxx has been cancelled because it went above our purchase limits for the following items:
40170 LEGO® City Build My City Acc.., 40307 Castle Interior Kit
We sometimes put limits on popular sets so as many LEGO fans as possible can get their hands on them. Although you won’t be able to buy those sets right now,
we regularly change and remove purchase limits, so it’s a good idea to keep an eye on our website over the next few months.
If there were other items in your order these will also have been cancelled, but you can reorder these through LEGO Shop at any time.
Kind regards,
LEGO® Customer Service
P.S. Please don’t reply to this email because your message won’t be accepted by our system.
EDIT: BTW ... this was in April, and several months after my first batch of Castle Accessory Sets for $2.49, so to my original point - these have been on sale for 75% off for a LOOOOOOOOOOOONG time (since the beginning of 2019).