I can find the documentation for Router OS v7 here: https://help.mikrotik.com/docs/display/ROS/RouterOS
Is this still the documentation for v6? https://wiki.mikrotik.com/wiki/Manual:TOC - is it still updated as v6 isn’t EOL?
v6 is very much EOL, no new features will be added.
Okay, I can accept no new features are been added but they’re still fixing bugs so that’s not EOL IMO. Bugs appear in documentation as much as software so are they still updating the documentation if it’s wrong? They must fix major security bugs for at least another 5 years? I’m guessing there are millions of ROS v6 devices out there.
No, bugs are also not fixed, except if they are security related. Please just use v7. It is stable.
So can ROS v7 now run on all older devices? I know you’ve recently split the wireless packages down, I assume to fit on low-memory devices?
What is your question? The only not-supported devices are SMIPS CPU based, which are very old. Documentation is not changing, because software is not changing.
When software not changing it is still evolving.
The question is, for me: is wiki.mikrotik.com still an official documentation? Or is old wiki EOL and online just as an archive?
What is in the Wiki still is true for v6, which has not changed and has not evolved. Wiki is not changed, because v6 will not be changing.
Main documentation is https://help.mikrotik.com/docs/
Most concepts are broad concepts and do not apply strictly to one version. Some features are only present in v7, but most of the theory in new documentation also applies to v6.
I’m sure I read somewhere that some devices (not SMIPS) couldn’t or shouldn’t be upgraded because they didn’t have enough resources - can’t recall whether that was CPU, storage or RAM.
I’d be very surprised if the ROS v6 documentation was perfectly accurate 
No documentation I write is every 100% bug free…
Wasn’t that already answered?