BOOTCAMP no longer listed as Device in Finder, but can boot into it on startup
Solved - the answer is that Windows was in a hibernate state. I was able to take Windows out of this state and now BOOTCAMP appears. I will accept my own answer when I am allowed to. More details of solution below.

I downloaded a trial of Paragon Disk Manager which showed the BOOTCAMP partition and provided an option to mount it. On attempting to mount, a dialogue popped up saying that the Windows operating system was in hibernate state and that it could not be mounted by accessing the partition by read-only operations; some write options would be required. I think an option was provided to proceed, but instead I decided to boot into Windows from start up as usual and attempt to fully and properly shut it down (as this is the least risky approach; better to get Windows to sort itself out regarding fully shut down and out of hibernate mode rather than from externally (i.e via an app in MacOS). There were some routine Windows updates ran but I don't think this is relevant. So after having shut Windows down. I started up into MacOS and can now see BOOTCAMP in finder. I don't work for Paragon or have any financial or otherwise interest in them, but the tool served a good purpose in that I could attempt to mount the partition which lead me to the solution.

I've seen something like this with dual-boot Windows / Ubuntu (Linux) systems whereby Ubuntu warns of same message about Windows being in a hibernate state when attempting to mount the Windows partition. So from that I conclude that the issue is not rare (and fortunately remediable). Ubuntu warns that attempting to mount is risky as it means it would have to modify the partition in order to mount it. Again it's therefore better to boot into Windows and fully shut it down. Not really sure why Windows would ever be hibernate mode when shut down though, but a "power cycle" seemed to get it out of this mode and into a fully shut down state.

So that's solved. However as a separate perhaps unrelated point I am concerned about the gpt show: /dev/disk0: Suspicious MBR at sector 0 message seen when running sudo gpt -r show /dev/disk0 as stated above. Any tips would be welcome, but my machine functions perfectly fine so this does not appear to affect it but I'd like to resolve this message anyway - research so far doesn't give a definitive answer it seems, I may write another question.