Vlad Safari 14 had an option to show favicons, but scrolling tabs (default behavior in <14) was only a hidden setting. So it was an option between: scrolling tabs with text / collapsed tabs with favicons.
In Safari 15, there's an option to disable collapsing tabs, but the favicons option is gone, kind of replacing it. So you now have a choice between: scrolling tabs with icons / collapsed icons. I guess they've decided that few people wanted just text, or that those who set it that way only did it because it also enabled scrolling.