I have Swiftkey installed.
I get a notification all the time about choosing my keyboard. I can get it to go away for a short time by selecting
Settings -> Language & Keyboard -> Current keyboard
Then selecting Swiftkey. But the notification comes back 30-60 mins later (I haven't timed it).
How do I get this to stop showing up? There are instructions on the Swiftkey site that are either outdated or don't apply to my phone: the settings don't look like that.
The relevant AE answer is 5 years old.