As I am in the process of chopping the overlong phab Help page into manageable subpages

Hmm, which problem would that solve, apart from making it harder to Ctrl+F things on a single central page and destroying translations for non-English speakers?

I'm not a fan of the vagueness of titles such as "Working_with_Phabricator" or "Advanced_features".

When do I not "work with Phabricator" while I am on the Phabricator website?
What does "working with Phabricator" [not] include?

If I am on the central Phab Help page (which is linked from everywhere) and search for info about batch edits or favorites/bookmarks, I will not find any info if it's hidden on some separate "Advanced_features" subpage. I could only find it if I had the very same understanding and knowledge what's "advanced" as the author?

I don't understand yet what problem is supposed to be solved which outweighs the disadvantages.

I currently don't favor this.

We had discussed my revamping the help page for Phabricator, which at the time I thought we agreed needed some serious work, and you invited me to go in and make changes. These are the changes I propose. If you no longer want me to make this page more readable and usable as a technical writer, I'm happy to revert my changes and close this ticket.

Hmm... maybe we had a bit different aspects of "more readable" in mind, sorry. :-/ (I guess I had more editing content in mind than necessarily moving content as a first step.)
I'd love to better understand the advantages of moving content to subpages, I guess, given the disadvantages that I'm afraid of (see previous content).
And/or a bit more info why the help page is less "manageable" currently.
Maybe transclusion would be an option?

As always, it's a question of what the audience finds most usable. Personally, I think if the audience is likely to search for things using Ctrl-F then my input is not needed -- that means the audience is highly technical and already knows what they're looking for on the page. I will defer to your superior knowledge about the page's audience and revert my changes.

(I reinserted the "Creating an account" section into the main document with my clarifications and changes.)

Apart from Ctrl+F (might not be very popular, indeed), I'd hope some folks look at the ToC of that central help page: What exactly could I expect behind terms like "advanced" or "working with" when I'd like to find instructions how I can perform a specific task that I have in mind and don't care if something is advanced or not? :)

There might be clear advantages of having separate subpages which might outweigh the disadvantages I've mentioned - if there are, please share them!