It has nothing to do with a relaxation of rules (again, a myth purported by some Traditionalists on CAF), as it is that the Church is very wise on remaining quiet about this issue. If the Church said you had to be at Mass for so-and-so time, it would encourage people to leave Mass early. People would think, “hey, I fulfilled the Sunday obligation so now I can leave”. People would end up thinking the remaining portion of Mass was redundant. What’s worse is that people would then use Church teaching to justify their actions - you would get what we call over here as “smart alecs”.
Therefore, the Church has kept quiet on this so as to allow the cultural influence to be the “de facto” rule. It is generally regarded in the Western world to be rude to leave Mass early. The Church has wisely hidden behind what is a cultural norm to implement what they believe is implicitly correct, even though for all intents and purposes, it is not a rule set in stone

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