Most users on Android likely have a third-party keyboard installed at this point, and with so many of them out there it's a safe bet that you can find one that personally fits your needs and what you're looking for. It isn't often that you find apps which can augment your third-party keyboard though. Generally you wouldn't need such a capability as many of the most popular keyboard apps are pretty full-featured, but if you feel that your keyboard app of choice needs just a little bit extra "something," an app called Inputting+ may just be the tool for you. It's an app that basically plugs into your third party keyboard of choice, whether it be Swiftkey, Fleksy, Swype or others, and gives you quick undo, redo actions with the tap of a button.
Say you're typing a status update in Facebook or sending out a message in your SMS app, and you want to undo a string of words. With Inputting+ you can do just by tapping the floating action button that appears overlayed on the screen when the app is installed. Tapping the button brings up an overlay panel which has the options for undoing, and redoing bits of text. If you use Fleksy, you already have a similar function to get rid of whole words by swiping from right to left on the keyboard, you don't however have a way to redo words you may have deleted but didn't mean to, short of re-typing them.
With Inputting+, when you open up certain apps, before you start typing you can tap on the Inputting+ floating action button to bring up the overlay panel then begin typing up the context. In this panel you can undo and redo as you go if you have the need. If not you can continue typing as normal and simply post or send. There is even an option for Inputting+ to save your input history, called Inputting Timeline, which collects all texts you typed up in any supported apps and copies them to a timeline so you can find and use those words easily later on. This is a pro feature though if you wish to have it which will set users back $0.99. Overall Inputting+ can be a pretty useful app, albeit with a couple of limitations. It only supports devices running on Android 5.0 Lollipop and above, and it doesn't currently support apps which use WebView, like Chrome for example. If you can get around those two things, checking out this free keyboard augmentation might be worth your time.