Back when Sun ran OpenOffice.org there was a (semi-)official manual of
style for the project.
* Is that manual of style still adhered to?
I don't know, but Sophie may know. We never really used it for the
user guides, but I believe it was used for writing Help.
* What is/was it called?
* Where can it be obtained from?
Here is one I remember.
https://wiki.openoffice.org/wiki/Documentation/Dashboard/Help_Style_Guide
I am not referring to the style manual within the documentation template
that Jean constructed/maintains.
There is also a short (and somewhat out of date) writing style guide
for LO user guides. It is Chapter 5 in the Contributors Guide. See
here: https://wiki.documentfoundation.org/Documentation/Development#Contributors_Guide
In Docs, we mainly referred to the relevant sections of _Read Me
First! A Style Guide for the Computer Industry_ by Sun Technical
Publications. I realise that isn't what you're looking for, but I
mention for completeness regarding semi-official manuals of style.
--Jean