https://thecritic.co.uk/the-left-wing-bias-of-wikipedia/
https://www.allsides.com/blog/wikipedia-biased
As to the claim that Wikipedia articles are 'well-sourced and fairly reliable' this is true for articles which are not in any way related to politics but misleading or sometimes even downright false where it concerns politically charged subjects, partly due to the (ab)use of the 'Perennial Sources' list which in for the most only allows sources which abide to the 'progressive' narrative. By banning sources which do not follow the narrative it is difficult to sometimes impossible to add corrections to biased articles since those corrections relate to facts only published on such sites. This often leads to lengthy discussions on the Talk pages for those articles where editors defend their deletion of such corrections by claiming the sources are banned or untrustworthy - this based on the Perennial Sources list [1] which lists heavily biased propagandist sources like MSNBC, ABC, CNN, Al Jazeera, SPLC, ADL, the Atlantic and many others as 'reliable'. In this way Wikipedia treats sources like pseudo-democratic countries treat parties: just ban the parties and candidates which don't tow the line and let the world marvel at your party or candidate 'winning' every election 'fair and square'.
The problem with Wikipedia's bias is very well known, just point a search engine at the question 'does wikipedia have a political bias' (or something along those lines) for examples. Articles on politically charged subjects - and there seem to be more and more of those - end up like talking points for the desired narrative instead of informative overviews of the subject matter. Such articles are then used as 'proof' of the position espoused in the narrative, as 'teaching material' to further the narrative, as input for LLM training runs which leads to those models ending up being more biased and more.
Yes, Wikipedia has a significant political/ideological bias towards the 'left' or 'progressive' side.
As to there being something 'left-wing' about a 'free, common, public source of information that is accessible and editable by anyone' I beg to differ. I consider this to be neither left-wing nor right-wing, liberal nor libertarian. Where ideology comes in to play is when someone with an ideological bias starts removing viewpoints which do not fit some desired narrative, especially when this is done in an organised fashion so as to crowd out dissenting viewpoints. As it stands now on Wikipedia it is those with a left-wing bias who are largely responsible for such activities. I won't claim that there is something inherently left-wing about the desire to silence those with dissenting views because I know this not to be true. I do claim this is inherently authoritarian and that this behaviour is what has dragged down Wikipedia. It would be a good thing for those in charge of keeping the project alive to realise that it stands or falls by its general reliability which has been severely affected by the actions of these activist editors.
A long-ish answer but that's because I've been around Wikipedia for a long time - since its inception - and consider it a damn shame that the project has been subject to ideological capture.
[1] https://en.wikipedia.org/wiki/Wikipedia:Reliable_sources/Per...