I have the same issue, i looked in the header.php in the editor and used the find function and nothing comes up. I am a bit surprised this is not standard to enter in an alt text for your header since its much needed for SEO. Any way to do it with updates that have been done since?
Also if theme updates and the changes are made. Does it wipe out the change?
I think this is an important issue that needs to be fixed
SEMRUSH is reporting it as a serious issue that should be addressed.