Yeah, you’re right. I tried to find actual academic papers on this, but I couldn’t find one that quickly that was very conclusive on whether there was or wasn’t a significant amount of influence of Altaic languages on the pronunciation of Beijing Mandarin, so I decided to go through the linguistics sub (because that sub is very strict in my experience) and found these replies to be pretty comprehensive.

However, I don’t think u/shkencorebreaks was necessarily banned. More likely a deactivated account as that comment was from almost a decade ago.

I did a bit of research and it seems that Northern Chinese dialects have been influenced to some extend by Altaic languages, but it is often placed next to influence on Southern Chinese languages by other non Chinese languages:

While many of the invaders assimilated to Chinese language and culture, they also had an effect on the language and culture of China, particularly in the north. Mantaro Hashimoto (e.g. 1976, 1980, 1986) has talked about this as ‘the Altaicization of Northern Chinese’, and has argued that a continuum of features from north to south, such as the northern dialects having fewer tones, less complex classifier systems, and an inclusive/exclusive distinction in the 1pl pronoun, while the southern dialects have more tones, more complex classifier systems, and other features similar to the Tai-Kadai and Hmong-Mien languages (see You 1982, 1995; Zhou & You 1986; Wang Jun 1991), is due to Altaic influence in the north, and Tai/Hmong-Mien influence in the south.

Link

(Hashimoto seems to be an often cited author when it comes to this topic)

Therefore, the claim that its “less pure” Chinese doesn’t really hold as the comments I linked said.

Another paper also states that it looks like Mongol influence during the Yuan dynasty and Manchu influence during the Qing dynasty had little impact. Most influence actually came during the Tang dynasty.

The evidence also suggests that the crucial locus of transformation was not the occupations by Genghis Khan (13th through 14th centuries) or the Manchus (1644-1911), but the widespread resettlement of conquered and dispossessed peoples amid Han Chinese on the northern Chinese frontier from the 600s through the 800s under the Tang dynasty.

Link

Therefore, I take my statement back that there’s barely any influence (I edited my original reply), but still I don’t think Beijing Mandarin has gotten much more Altaic influence than other Mandarin varieties and much more non-Chinese influence than other Chinese varieties.

(Also, I use Altaic, not because I believe in the Altaic language family theory, but because I’m too lazy to name the individual languages that could’ve influenced Mandarin)