OTOH, it wouldn't have been UTF-8 -- it would have been an EBCDIC-8 thing, and probably not good :)
In some small decisions EBCDIC makes as much or more sense than ASCII; the decades of problems have been that ASCII and EBCDIC coexisted from basically the beginning. (IBM could have delayed the System/360 while ASCII was standardized and likely have saved decades of programmer grief.) The reasons that UTF-EBCDIC is so bad (such as that it is always a 16-bit encoding) could likely have been avoided had IBM awareness of UTF-8 ahead of time.
Maybe if IBM had something like UTF-8 as far back as the 1930s, AT&T needing backward compatibility with their teleprinters might not have been as big of a deal and ASCII might have been more IBM dominated. Or since this is a sci-fi scenario, you just impress on IBM that they need to build a telegraph compatible teleprinter model or three in addition to all their focus on punch cards, and maybe they'd get all of it interoperable themselves ahead of ASCII.
Though that starts to ask about the scenario what happens if you give UTF-8 to early Baudot code developers in the telegraph world. You might have a hard time to convince them they need more than 5-bits, but if you could accomplish that, imagine where telegraphy could have gone. Winking face emoji full stop
I think this points to why the science fiction scenario really is a science fiction scenario -- I think decoding and interpreting UTF-8, using it to control, say, a teleprompter, is probably significantly enough more expensive than ASCII that it would have been a no go, too hard/expensive or entirely implausible to implement in a teleprompter using even 1960s digital technology.
"Sigh, another swiss cheese card jammed the reader from all these emojis."