> (There’s precisely one thing like this I’m aware of terminal emulators adding in recent memory: clickable links, ca. six years ago.)
And they are often disabled by default, as a potential security risk. We don't get to have fun things, do we? (also worth read: CVE-2003-0063, abusing escape seq is unfortunately a valid concern against adding more stuff).
On the other hand, more and more emulators are adding support for various graphic protocols (sixel, iTerm2 format, kitty format).
> I’ve been waiting for one or more terminal emulators to get together and add some ridiculous new escape codes[...]
Well, it's not much, but mintty apparently has some interesting stuff like audio support[0], and codes for font size and font family[1][2].
iTerm2 also has a bunch of custom escape sequences of varying level of usefulness starting from displaying fireworks animation on cursor position to sending system notifications[3] (although sadly I could not get the last one to work for me).
For some semblance of forms, you can check bubbles[4] and gum[5] (binary to easily incorporate the components into shell scripts).
There is "Background for Unicode consideration of Cistercian numerals" (https://www.unicode.org/L2/L2020/20290-cistercian-digits.pdf)
And also one in the Under-ConScript Unicode Registry for the Private Use Area (https://www.kreativekorp.com/ucsur/) (https://www.kreativekorp.com/ucsur/charts/cistercian.html)