(If you go to "fake science" and "conspiracy theory" website, I am sure you can find some. 🐵
Maybe you should be more clear what you mean by "high frequency" because 5G can have many frequencies. Maybe you talk about millimeter waves. If anything, they have significantly less penetration into the brain because of a phenomenon called skin depth, which prevents these frequencies from entering deep into most solid and liquid objects of higher dielectric. This also makes the higher frequencies unsuitable for mobile applications and will be used mostly for line of sight fixed wireless, i.e nowhere near your head. Also technologies like beam forming will lower the overall exposure. All these frequencies are in the microwave region, orders of magnitude lower than even infrared, so standing next to a campfire will expose you to significantly more "radiation" and at higher frequencies than any communication ever could. These are not dangerous ionizing radiations such as UV or X-ray or similar.
Of course there is the higher information density due to the higher speed and lower latency. Typical brains don't want to wait for information forever and want everything available as fast as possible, so the higher speed is beneficial to the mental state. 😄
I want to think there are already some studies about that... I used Google Scholar to search for this and found EMF Exposure Study Concerning mmWave Phased Array in Mobile Devices for 5G Communication in the first page. I hope it gives you more references to move forward with your research.
Something else about 5G: just recently they were discussing the real concern that 5G radiation could interfere with weather satellite communications, reducing our ability to make accurate forecasts.