The edit did exist. So the phrase "Multiple studies have found a left-wing bias at Wikipedia" can be found last in the edit (4 February 2025) [1] and was removed by the editor Aquillion with the explanation [2]
Not seeing anything in this source that supports this language; they summarize no other sources that I could see, and their own conclusions are more complex than this (as covered further down the article.)
I think that the one who previously introduced the phrase should have either not stated 'Multiple studies' or provided information about other studies. I suspect that a single research is usually not enough to be mentioned in an article
[1] https://en.wikipedia.org/w/index.php?title=Ideological_bias_...
[2] https://en.wikipedia.org/w/index.php?title=Ideological_bias_...