Seen Blocker for Chrome
More About Seen Blocker
This extension is small and easy to use - two options are available which toggle the 'Seen' and 'Typing' blocking, and can be turned on and off at your leisure.
Note: This does NOT block the 'Seen' feature from Facebook group posts, only for messaging.
Source available on GitHub: https://github.com/MicahNus/Seen-Blocker