Until when should one be concerned about chametz that has passed over Pesach

After Pesach, everyone talks about the "code," and in the supermarket, everyone turns the packages and does "matzev"... Much time has passed, the public has returned to routine, but the retail world has not changed. Half of the products are still produced before Pesach. I asked people, and everyone repeats the date "Shavuot"! After Shavuot, it is customary not to worry. Am I being too stringent by avoiding it now? Perhaps I am correct, and this is the main rule?

Hello,

According to the law, there is no difference between before the festival of Shavuot and after the festival. 

If it is known that chametz has passed Pesach in prohibition, it is forbidden forever. Therefore, if one does not rely on the sale, it must be forbidden forever.

Indeed, there are those who did not worry after the festival of Shavuot, and this is for two reasons.

A. If there is doubt whether it passed Pesach, it is permitted. After a long time from Pesach, there is no longer certainty, only doubt. [However, for products with a production date, this reason does not apply].

B. Some of the great Chassidim prohibited chametz that was sold only as a stringency and not as a prohibition, and they were stringent about this only until the festival of Shavuot, due to the greatness of the days 

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