It seems to me that being fact-oriented, especially when preferring facts from authorities and institutions, tends to get a source labeled as liberal-biased.
The trouble is that even if these facts are unambiguously correct, their reporting can be misleading, by interpretation, implication, conjecture, selective reporting, and/or omission. That's how bias is injected even into "factual" reporting, and why reality's "liberal bias" may just be a reflection of the predominant institutional bias in popular media.
All Sides has an article noting the left leaning bias of Wikipedia, including references to five different studies. It also mentions how the cofounder of Wikipedia has called out the site’s liberal bias: https://www.allsides.com/blog/wikipedia-biased
Personally, I see it in articles on any topic that is controversial, particularly along the axis of the left-right divide in America. Wikipedia is a theater for activism/ideology just like every other space (like our work places) has been opted into the same war in the last 5 years or so. It is most apparent to me when I read an article about influential or popular figures who are moderate or right wing.
No, not every conjecture has been researched. But if you're at all familiar with editorial policy on wikipedia, you'll know that only mainstream sources are typically treated as WP:reliablesource (e.g. wapo, huffpo, cnn, nbc, occasionally salon and vox tier) and the vast majority of right leaning sources (aside from fox) are effectively blacklisted. Not to mention the regular editorial bullying on politically sensitive pages that reinforces the bias by chasing away neutral or non-progressive leaning editors with alternative points of view.
Also if the allegation is that wikipedia has an editorial bias, I wouldn't necessarily trust wikipedia's own evaluation of it's bias.
> … the vast majority of right leaning sources (aside from fox) are effectively blacklisted.
This reminds me of a past blog post, a few weeks ago or so, from a journalist who spoke about how strongly right leaning "journalists" often think of their writing as political influencing and counter-acting the left journalism.
I don't know how true that really is but such a tendency could be a good faith explanation for such a rule (as long as it has exceptions).
Do you have a robust academic citation for this?
None of the sources here (https://en.m.wikipedia.org/wiki/Ideological_bias_on_Wikipedi...) suggest a heavy bias, and overall it seems that bias is attenuated with stronger participation.
I haven't been able to find sources other than those linked.