Since the recent WordPress auto update incident left all twenty-* default themes in my install, here's a gif how the look changed over the last ten years…

Time flies, eh? And yes, there's no "TwentyEighteen" Theme - thanks to the development of the Gutenberg editor, which took longer as expected at first and so the "TwentyNineteen" theme took over in late 2018.

I'm surprised how different the "TwentyFourteen" looks, but I cannot remember that I build a site back then based on that theme.

The "TwentyEleven" looks still most mature to me.