My iPhone 6sPlus was having this same battery drain issue after installing IOS 14.2. I fully charged my phone and went to bed with the battery at 100% and when I woke up in the morning it was down to 1%. I had previously tried all the suggestions offered that I could find in a Google search. I did NOT try to bring my phone back to default factory settings and then have to reload everything on my phone because I did not want to go through all of that. But I think I finally found something that worked. What I did was I went to Settings, Battery and toggled OFF "Battery Percentage." Then I scrolled down and selected Battery Health, and toggled OFF Optimized Battery Charging. Then I did a hard restart of the phone, went back into the Battery settings again and turned Optimized Battery Charging back ON and Battery Percentage back ON. My battery percentage indicator immediately went from 1% to 79% after that. I think the upgrade must have caused a glitch with the Battery Health software on the phone and this seemed to fix it. I will know for sure tomorrow morning. If all is still good at that time, I will feel confident that my battery drain problem has been fixed.