How to disable auto-capitalization in Apple Mail app (really impossible?) I am on an iPad Pro running iPad OS 13.2.3 and using the included Mail app from Apple.

I have disabled auto-capitaliztion globally by going to Settings -> General -> Keyboard and flipping the switch next to "Auto-capitalization". However, it is still auto-capitalizing inside the Mail app. This behavior has continued after force-quitting the app, restarting the iPad and reopening the app.

Apple Support is telling me that it is impossible to disable auto-capitalization after a period within the Mail app. I am a biologist and need to be able to type things like "E. coli" without the "c" being capitalized. Is this truly impossible? Do I need to use a third-party Mail app just for this?
Go to Settings → Keyboard and set up a text replacement for ecoli → ecoli to see if it gets suppressed. Though it is not a viable solution for all of your words.

Look into Grammarly app where you can type, maintain a personal dictionary of corrections and then use gestures to quickly copy paste or even run them in split mode.