That's the microphone for Google Assistant. It allows you to do google queries by voice. You can Disable it in Settings > Apps

 

That's the microphone for Google Assistant. It allows you to do google queries by voice. You can Disable it in Settings > Apps

I know. Assistant is disabled but the microphone is still there.

 

It took some digging because Google assistant is not listed under Settings/apps on my Samsung Tab A, running Oreo (I haven't checked my Samsung phone yet; its running Marshmallow).

It is listed under Settings --> Google
Then Search, Assistant and Voice
After that I had to select Google Assistant in the left column.
Then there were 4 choices You, Assistant, Services and Home.
I chose Assistant. Below all the options I saw Assistant devices and finally tapped on Tablet.
Phew, finally I was able to disable it.

And after all that, the microphone still shows on the Home screen and says "Say Ok Google" in the search bar so nothing has changed!

About the only thing I can do is long press it then I have the option to remove the google search bar with the microphone from the Home screen.

 

Thank's mate. I am running Android P and also tried everything you have described. Microphone is still there I guess that Google made it impossible to remove. Regarding home screen I installed "SearchBar Ex" and made a custom search bar without a microphone. I'll mark this topic solved because it's obvious that we can't do anything about it (without root) .

 

This is my home screen search bar.

You can choose size, icon, background ... It's compatible with Nova launcher(probably with most of them) .