[noparse][/noparse] becoming [plain][/plain] was one of the changes which arrived when we moved from the previous to XenForo forum software.
All the [noparse][/noparse] message tags were converted to [plain][/plain] when we migrated, but not the text within them. So the instructions to use [noparse][/noparse] remain and are of course now wrong, do not work. I guess you saw one of those older posts which said to use [noparse][/noparse].
Not sure what is best or easiest to do; find and update those old posts or try and get [noparse][/noparse] accepted as valid code tags.
"PLAIN" just doesn't describe the action wanted. "NOPARSE" is technically more descriptive, but not exactly intuitive. I've had to look both of them up in the help, as well as "[CODE rich]", whenever I wanted to use that functionality.
At the very least, I would think adding some BBCode to that "Insert" drop down would be convenient. Perhaps replace the highly useful D) "Spoiler", "Inline spoiler" and "_ Inline code" menu items with "NOPARSE" and "Code Rich"?
As far as the text conversion is concerned, I did a quick search on "[NOPARSE]" and found only seven occurances since the year 2000. Some of those were changes within [CODE][/CODE] tags. Didn't understand that. That's not a huge problem.