Since I hated Gboard since it's rename, I wanted the good old AOSP Keyboard back. I couldn't find any .apk of newest version, so I extracted it from latest CM14.1 build of OnePlus 3 and made a flashable zip. I also applied a dark theme and set it as default, looks nice when booting with a encrypted data partition.
How to install:
1. download zip
2. flash through TWRP
3. you may also want to delete gboard system app (/system/app/LatinImeGoogle) and uninstall it from Play Store if still listed
I only tested it on OxygenOS OB13 and OB14, but should work on other ROMs too.
*** USE AT YOUR OWN RISK! ***