For nested templates, when did `>>` become standard C++ (instead of `> >`)?

I seem to recall, in times of yore, being warned against putting two > characters right next to each other (without a space) when dealing with nested template parameters. I even vaguely remember declaring vectors of vectors of whatever and encountering this compilation error.

But now I find that there is absolutely nothing wrong with compiling the dreaded >>...

My question(s) are thus:

At what point did this convention become an acceptable practice?

Is it part of standard C++?

Was it always part of the standard and the compilers I used (and the professors I had) in college just didn't support it yet?

Maybe these questions are a tad bit historical, but for me it seems that proper historical context makes actual remembering trivial.