"New attempt, now I got a "phabricator-production would like to have basic access on your behalf on all projects of this site" confirmation request, and declined, because I didn't know why something wants to impersonate my account."
I'm attaching a screenshot.
While I had created two Phabricator accounts accepting that dialog without even reading it properly, I reckon that such text is unclear and likely to scare off (or simply drive away) many users.
It sounds as if phabricator-production would edit wiki pages and what not on your name, whan actually the only thing it wants to do is to read your username and perhaps your email address (need to confirm this), and it will not touch anything "on your behalf on all projects of this site".
Can we provide something closer to the truth, as mobile apps are getting us used?
$provider would like to access to your username (and anything else it is accessing as well) in this site.