Arguably, the last date of the "times of single-digit Windows" would be one day before the release of Win 10, which was on 2015-07-29.

Didn't Win 8 have fewer options for adapting the UI compared to previous versions of Windows?

I was indeed referring to Windows 3/3.1/3.11/NT 3.5. Wasn’t sure of another term that would capture that era in one word.

Decimalized? Rational number Windows?

early 90s Windows?

Pre 95? 16-bit?

I don't know how far back they mean, but Im pretty sure I recall it being in XP.