I think it is, personally. Mostly because common UI appearance and paradigms makes software easier to use, especially for non-technical users.

But in this specific instance I was referring to OP’s claim that it’s impossible to identify many Flutter apps, which I dispute.

I think people might be talking past each other on this issue. If you are on one platform only, then yes having apps behave the same is nice for muscle memory. However, if you're using multiple devices and platforms, then I want Slack to work the same regardless of device, I don't want iOS native navigation on the iPhone and Windows native navigation on the laptop.