Indeed I was looking at that. We use input-group-button because it is the way Bootstrap propose for this. Now it is possible to have custom CSS to make it works like the form-control-feedback.
We can not use form-control-feedback because it works only for one icon on the right and in Bootstrap 4 this has been replaced by standard HTML5 validation.
But the big issue with custom CSS is that it may break bootstrap themes if it is not done correctly. The solution without bootstrap seems to be a good starting point but it should work with img (for New theme icon for Tryton) and with image on both sides.