Noticing the size of the mermaid plugin and given that I upgrade via a 540k connection at best I didn't upload it, with no apparent problems.

However I have since found that it is mentioned in the [/plugins/tinymice.plugin.php] version 7.1.7 but was not in the earlier version 6.10.5

I was comparing the files as the older version worked better than the newer. This came to light whilst looking at the problem presented at:
https://forums.b2evolution.net/text-area-missing-from-comment-form-when-logged

So I searched for 'mermaid' in a) the forums and b) the docs and did not find a single reference. ~ so here's one

Is it a core plugin that other plugins and aspects of b2 rely on??