A lot of people seem to be removing Cydia Installer either by mistake or purposefully, only to realize it is very hard to get it back once you do so. Uninstalling Cydia is unfortunately, definitely possible and easy to do, but the fixes to get it back working on the device are usually not working due to them being outdated, or due to the permissions that have changed since the fixes were posted.
If you have removed Cydia, your Jailbreak still works, the kernel is still patched so you can still use the already installed tweaks, themes and patches, but there is no way you can install additional things, or manage your repos anymore. To fix this, we're going to use a File Manager on the device, being it either Filza or iFile.
This method that I've discovered and I've demoed in a Reddit post involves removing yalu's installation file to force Yalu to unpack its contents again. It see, to be working pretty well on all devices I have tested.
My Reddit post: https://redd.it/5yowqh
Cydia Extender – Sign IPA from iPhone: https://youtu.be/K6At_IPjxkY
How to fix Cydia Impactor Errors: https://youtu.be/5CgEAgKc8z8
How to Jailbreak iOS 10 – 10.2: https://youtu.be/5Sfg-3z4Teg
25PP Jailbreak: https://youtu.be/wfuq0xBiubc
iOS 9.3.5 Untethered Jailbreak: https://youtu.be/3lGMppIcOyU