Reviewing the existing answers, none of them actually address the question from the perspective of D&D 5e. So the simple solution to the "multiple systems" problem is to just remove dnd-5e and dungeons-and-dragons. This would not invalidate any of the answers. Further, we can direct OP to this question for how it is handled in 5e: What rolls should players get to see?. If we do this, answers to everything they asked for are still accessible to them, but we have a properly scoped question.
As for the concern about the question being opinion based, it anticipates the issue by phrasing the question accordingly (emphasis mine):
Obviously this is largely opinion-based, and I don't know if you treat that like Stack Overflow does, so I'll leave an objective question here: is hiding DM dice rolls from the players a standard practice according to game rules?
Whether or not the rules address this problem is not opinion based.
Some of the answers do have quality concerns.
Several very poor answers were given for this question, which probably contributed to its closure as opinion based. These answers probably need some attention, warranting either deletion for providing a terse opinion statement with no helpful discussion, or a post notice where insufficient support is provided for whatever potentially helpful claim is offered.