I experienced the red battery screen of death on my iPad 6. I live 150 miles from an Apple Store, so could not easily get a battery test. Took it to Best Buy. They said ship it back to Apple and I might get an answer on the battery in a few weeks.
This problem had been occurring sporadically over the last year. The iPad is 3 years old and is running iOS 14.4.2. On previous occasions the iPad self corrected after a few minutes or hours. I could not tell why.
However, on this last occasion the screen went dead and I got the dreaded red battery that would not go away. I tried the recommended double button push multiple times over several days. Nothing. Finally, I thought maybe it was a temperature problem. So, I cold soaked the iPad in the refrigerator at about 40 deg(F) for approximately three hours. I pulled it out. Still no response to the double button press routine. I then immediately decided to heat it up after taking it out of the refrigerator. I did this by putting a gel wrap (similar to what you would use to wrap a knee) into the microwave for a minute. I then laid a dish towel on the kitchen counter and placed the heated gel pack on that. I then placed the iPad on top of the gel pad and connected it to the charger. Voila, after about 15 minutes I opened the iPad magnetic cover to find that my Home Screen magically appeared in all its glory.
I looked at the battery history after it turned on and saw that it was fully charged when I put I in the refrigerator. It then discharged over a period of several hours to near zero while in the refrigerator and in the sleep mode. Then, it miraculously re-charged to 100% in a matter of minutes after I took it out of the refrigerator and placed it on the heated gel pack. I doubt that the battery could charge that fast. It appears there may be something wrong with the state of charge indication hardware/software. Who knows?? Maybe Apple does, but they are not saying.