In all those examples (SIM, SMS, 5G, RCS, LTE), the long version is a technical phrase that has no inherent meaning. As such, the acronyms are just as descriptive as the long versions. For example, in 5G, we're basically being told that it is newer than 4G or 3G. We don't actually know the difference between 5G and 4G other than higher numbers are newer. And we don't really need to know anything more than that. So it made sense to make short forms of these. It's easier to learn and use the short form than the long form, with no real loss of meaning.
Note that I said SIM rather than eSIM. I'm fairly certain that most people who see eSIM ignore the e and just read the SIM. Same thing with the MMS in SMS/MMS. But that's a side issue.
Similarly, what does User Experience mean? People in the field know that it means understanding how the user interface (UI) fits into the user's workflow (or overall eXperience). But people outside the field won't find User Experience any clearer than UX. Both require about the same knowledge to understand. UI falls on the other side. User Interface is clearer, so UI should only be used without explanation when it's reasonable to assume that the reader already knows what it means. I.e. it should not have been used without explanation in this thread.
By contrast, One-Time Password is not a technical term and is understandable. Rendering it as OTP makes it less understandable. And more ambiguous. On The Phone is an another long form and makes just as much sense. One True Pairing also makes sense until you find out what it's actually supposed to mean.
Because One-Time Password is not a technical term, OTP will never become superior to it. It will always be at least as clear, if not clearer, to write it out. And of course, one can always write One-Time Password (OTP) or OTP (One-Time Password) and get the benefits of both.
The message in the phone should write out One-Time Password. And for a better user experience, it would make sense to only show that message on the screen that shows an actual One-Time Password. Currently it shows on the main text screen which doesn't provide sufficient context to understand it, even for someone like me who generates the stupid things programmatically. I.e. I would actually be considered in that field (broadly) and I still didn't automatically recognize that acronym. Perhaps if I were more mobile-focused (rather than PC-focused), but I'm not.
There's also a bit of a question about commonality. However, I find it very unlikely that OTP will ever become as common as, say, NASA. NASA's long form is slightly clearer (because it indicates that S is for Space), but it is widely enough known that that isn't really necessary. NASA is the space agency. What the NAA represent is not important (National Aeronautics and Space Administration). FBI and CIA too. Or even GM and GE. But the typical interaction with an OTP does not require the acronym. They don't say, "Enter the OTP." They say, "Enter the code we texted to your phone." So most people will never become familiar enough with the acronym to translate it to its actual meaning.
So if you're asking where the line is, I would say between OTP and SIM or between UI and UX. Outside technical contexts, OTP and UI should be written out. And OTP is farther from the line than UI, which at least has some hope of becoming common enough to be recognizable outside technical contexts (like NASA is).
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