It is located at /themes/forum_themes/eebootstrap/images/marker_old_topic.gif

Just looked at the source code of my forums and that is repeating there. So when theme should use only font-awesome, can i override that gif, and make it plain blank white?

Thanks in advance.

it’s defualt hardcoded image by Forum Module.
you can override, but in case of JS disabled NO images are displayed.

If you disable JS in browser you can view all the default icons.

Please use the correct forum next time, thank you.