How do I get the "Change keyboard" notification to go away for more than an hour?
As of now all i can see is there are only 2 possible solutions 
1) keep only one keyboard as suggested in 5 year old question 
2) or keep all keyboards but disable all except for the one that you use by going to setting->language &amp; input->virtual keyboard->manage keyboards, here disable all except one that you use regularly  
Tried and tested on stock nougat 7.0 on moto g5+