Can your terminal do emojis? How big?

GP explained already. Grapheme clusters ≠ glyphs. To find the number of glyphs you need the font.

An emoji can render as one or two or three or more glyphs depending on what font the user has installed, because many emoji are formed by joining two or more emoji by a ZWJ)

(Also even in a monospace font not all glyphs are of ﷽ equal width)

And you are just describing bugs. This is not just an emoji issue: it will also fail to render CJK characters.

And how the program that prints such emojis should deal with this? Like, how should e.g. readline handle the user pressing the Backspace key after inputting such an emoji after prompted for input? It needs to know precisely how many lines and columns user's input takes: a huge chunk of code in that library is devoted precisely to this, because simply emitting "\b \b" doesn't work.

And if the user opens the terminal emulator's settings and changes the font, should the program be sent some signal to redraw the window, as it happens when the window size changes? E.g. that emoji was in a 10-columns wide edit field and so characters after it fit when that emoji was 1 column wide, but now it's 2 columns wide, so the ncurses should now trim the last character in that field.

Or try this funny little experiment, for instance: resize your terminal to something like 30 cols by 5 rows and run "script -c bash temrinal_log.txt". Now hold "a" key until you enter enough "a"s that the shell prompt is no longer visible. Now hold Backspace until you've erased all "a" and cursor no longer moves. What do you see on the screen? Now press Ctrl-D to exit the "script" session, and study the transcript in temrinal_log.txt in a hex editor. Ponder on the mechanisms that bash (readline inside it, really) uses to implement line-editing.