Not all 2018 have MCU/2. As stated above, it was around March 2018 when MCU/2 came out.
Also note, that Sentry mode needs to have a USB to be activated according to Elon's tweet that I published above. While not necessarily written into the Model S and Model X Release Notes, if you have AP2.5 and MCU/2, you should be able to plug in the USB and see the option appear.
I haven't seen any confirmation with 2019.8.x that Sentry mode is available for MCU/1 regardless if you have AP2 or AP2.5.
I guess we'll find out tomorrow once 20918.8.x starts rolling out beyond Early Access Participants.