The UN has some guidelines for gender-inclusive language used for their staff.
They do say that plural pronouns are a bad thing for formal writing (though OK for informal writing, such as personal emails):
In informal writing, such as emails, plural pronouns may be used as a
shortcut to ensure gender inclusiveness. Such strategies are not
recommended in formal writing.
For more official sources of writers and language experts, the American Heritage Dictionary has a panel of experts who are polled on language changes. "singular they" was added as a usage (2b) referring to gender-neutral writing, but polling showed controversary whether it was correct use of language:
Resistance remains strongest when the sentence refers to a specific
individual whose gender is unknown, rather than to a generic
individual representative of anyone: in our 2015 survey, 58 percent of
the Panel found We thank the anonymous reviewer for their helpful
The recent use of singular they for a known person who identifies as
neither male nor female remains controversial; as of 2015 only 27
percent of the Panelists accepted Scout was born male, but now they do
not identify as either traditional gender.
My opinion is that both "they" and "he" should be acceptable. If the majority here have to be nice enough to accept the LGBTQ+ definition of gender pronouns, then then LGBTQ+ group should also be nice enough to accept that many use 'generic he' in a gender-inclusive manner. Tweaking all posts and comments to use username or OP is not going to be practical, nobody realistically writes that way, so forcing a change in their manner of thinking is going to be problematic, and that's not even before we consider people for whom English is not their primary language.
I have referred to someone on this site as "he" without knowing anything about the author other than their username. I received a reply telling me that they were flagging my comment because I hadn't referred to them "correctly", which caused me great distress. I'd rather the site was primarily concerned with intellectual responses on a totally genderless basis only.