So, I just scrolled all the way back in a couple of chats and found that the earliest available message was mid-conversation. Everything before that is gone. It seems Whatsapp has been silently deleting old messages. This is highly disturbing to me because the oldest period of messages available right now relate to the last months of my now-deceased mother. A lot of very important messages pertaining to that period are now gone!
Automatic backups to Google Drive have been on, but I just found that Google Drive won't let me download them on my own. The only options it gives me are to turn off the updates or delete the backup. It really pisses me off that Google gives Whatsapp more privilege to access my data than me.
I tried the export chat feature of Whatsapp, but it doesn't export everything. In my most important chat, I can scroll all the way back to September 2017, but the exported file only has from December 13, 2018, a week ago, onwards.
I tried the local backup, the Whatsapp directory in internal storage, but everything there is encrypted! What's it encrypted for, I wonder? So the user can't use it outside of Whatsapp! Great work Whatsapp team! I'm about to flip my bolted down desk!
It seems the only option available to me is to screenshot everything manually from last year onwards (that's gotta be thousands of messages to scroll through and screenshot), but I hope that someone here can give me a better idea to get the plaintext. I don't shy away from complex solutions, and though I'm not an Android developer, I am a programmer with a lot of Linux experience.