If you use millennials (Get Y) as a characterization for people now in 20s and 30s, up to about 40, I think you'd find that a decent percentage of them have experienced rectal temps, though as others have noted it may well have been only when they were very small and too young to remember. With some exceptions, it seems that RT stopped at much earlier ages for Gen Y and later than it did for people like me in Gen X, or those who are older.
I did some medicine cabinet snooping (at friends' or cousins' houses, for example) in the 80s and 90s and still saw rectal thermometers, or even digital ones with "rectal" noted on them with a sticker or piece of tape.
I've done play exams with a fair number of millennials, and while most don't remember getting them, they seem to be ok with getting them and are at least aware that it's a route for taking temperatures. And some have mentioned that they remember that a younger sibling got them, but they don't remember getting them themselves but assume they did.
Certainly other settings where rectal temps might have been used before on older patients - like post-surgery - ear thermometers had already taken hold in the 80s (I had knee surgery in like 1987 and remember my dad making a joke about how they used to take temps in the recovery room when he came back to see me when I got out of surgery and the nurse put the sensor in my ear).
While slightly different, I do think millennials have probably had more consistent experiences with getting suppositories - e.g., for constipation for for Tylenol - while growing up.