I understand not every decade was a clean break, but each decade has fairly distinct defining properties. It feels like most of the 21st century has been a single run on with smaller changes. I know sometimes the definitions don’t come into focus until later, but I’ve been around since the 80s and I can distinctly remember the changes between the 80s, 90s and 00s as they were happening.
Buildings have become significantly more expensive to build, so only rich people build them in planned formats to make as much money as possible. Back in the day, regular people could build their own properties or afford to renovate according to their taste.
I think this is certainly one important element to it!
And when I think of America, I actually can see distinctions in 2020 / 2010s architecture compared to 2000s or earlier - America can afford to build where Britain and other european countries can’t, really. Or won’t, because of environmental reasons and lower population growth.