I learnt today that the JR pass doesn't allow one to travel using the fastest type of Shinkansens, namely Nozomi and Mizuho (running in the Tokaido-Sanyo-Kyushu lines). For reference see this webpage. I am very surprised to know this because I do not understand how do they manage to enforce this restriction. If you don't need a reservation then you can simply use the JR pass as an entry ticket into the shinkansen area and then you are free to ride any shinkansen (which has a non-reserved car). In my 2+ years of living in Japan I have never seen any officer in shinkansens manually checking tickets. So how do they manage to ensure that the tourists who are using JR pass are not traveling in the non-reserved cars of Nozomi and Mizuho? Is it just based on trust?
The comments by Hilmar and dda helped me get to the answer, so let me add a proper answer to my question.The thing is, the train conductors randomly checks tickets in the non-reserved cars to ensure such things don't happen. Yes, it's true that it is not a full proof mechanism to prevent such activities and some tourists can get away with it. To be fair, some of them might not be even aware of such restrictions in the first place. But if you have a JR pass then make sure you do not get on the Nozomi and Mizuho trains. These trains help the locals get to their destinations efficiently during business hours whereas tourists usually can spare a few more minutes during their leisure activities.
Also I would add that they are restructuring the JR pass prices and usage from October 2023. According to the new rule tourists can then avail the Nozomi or Mizuho with their JR pass with an additional fee.