Gee, if only we'd known about masks and lack of oxygen … Oh wait, we did.
But now there are new studies that are actually being published proving it:

In one of the studies referenced above, researchers determined a mere 0.3% prolonged exposure to carbon dioxide to young mice and pregnant rats caused brain damage, a decline in memory, and elevated anxiety.  A separate study determined male mice subjected to only 2.5% of carbon dioxide for four hours, equating to half a percent for people, caused the cells in the testes and even sperm to self-destruct.  Yet another study determined 3% carbon dioxide in rats carrying a baby resulted in birth defects and stillbirth.
German scientists pointed to the spike in human stillbirths that occurred throughout the pandemic, highlighting how evidence shows the use of masks may be tied to the significant spike of stillbirths worldwide, where the percentage increased from 28% to 33% in aggregate.  Making matters even worse is that the researchers found kids born during the years of the COVID-19 pandemic suffered two full standard deviation reductions in motor skills, verbal skills, and overarching cognitive performance.
Truthfully I'm not sure what to think about this.  Blaming stillbirths on the masks discounts the shots that doctors were giving to pregnant women without any proof whatsoever that it was safe for either the mother or the baby.
It is clear though that people who consistently wore masks (and are still wearing them) show signs of mental deficiency.
What can breathing too much carbon dioxide do to you? The authors write that “at levels between 0.05% and 0.5% CO2,” one might experience an “increased heart rate, increased blood pressure and overall increased circulation with the symptoms of headache, fatigue, difficulty concentrating, dizziness, rhinitis, and dry cough.” Rates above 0.5 percent can lead to “reduced cognitive performance, impaired decision-making and reduced speed of cognitive solutions.” Beyond 1 percent, “the harmful effects include respiratory acidosis, metabolic stress, increased blood flow and decreased exercise tolerance.” Again, mask-wearers are likely breathing in CO2 levels between 1.4 percent and 3.2 percent, well above any of these thresholds.
It might have been a feature, instead of a bug when the fear mongering started.  If people are wearing masks, then they would likely be more vulnerable  to the psychological pressure being put on by the media, government, and the co-opted medical profession.
And the CDC is still pushing masks for medical professionals.  Because we all want cognitively impaired folks handling our care, especially in hospital. Especially when masks don't do anything to prevent the transmission of respiratory viruses.

https://www.dailymail.co.uk/health/article-11982039/Face-masks-raise-risk-stillbirths-testicular-issues-cognitive-decline-study-says.html