The conventions are widely used and Python is used successfully in numerous “large monoliths”. Saying that the conventions may as well not exist if they’re not enforced is demonstrably nonsense.
It depends on your team. If the whole team "gets it" then things will be fine. But if you've got a team with juniors or people happy to do whatever crap they or ChatGPT can come up with to make things work then it doesn't.