I am not with Laminar Research; just a flight simmer.
My understanding is that once a new version is released there are no more updates available for the older version. This is also confirmed at the following link https://xplane.ultracartstore.com/checkout/singleLoad.do?r=1518566077769&merchantId=MEYER related to the purchase of version 9.
In my case the last update for XP10 was version 10.51 which ceased when XP11.0 was released having originally started with XP10.25
You may choose to contact Laminar Research at email@example.com for a more definative answer.