Really, rectal temperatures have become less and less common for a combination of several trends: technology changes, time constraints, growing prudishness (around medical things), and changes to how we treat children.
First, and most important, the technology has changed. Back when I was a child, it was all glass and mercury thermometers. To get a good oral temperature, you had to keep the thermometer in the right place, under your tongue, mouth closed for three or four minutes. And you couldn’t always tell if the patient had done this. Plus you had the risk of the thermometer being bitten and broken in the mouth. And under arm temperatures took even longer, so the problem was worse.
If you took a rectal temperature, you didn’t need the patient to cooperate beyond laying somewhat still. If you kept it inserted into the rectum for three or four minutes, you we going to get the right temperature. You wouldn’t chance missing a fever.
Back then, it made sense to take a temperature at an older age than we would today. Modern digital thermometers only have to stay in the right place for 10 or 15 seconds and give some indication if things went wrong.
These days, you can certainly trust most five year olds to keep the thermometer under their tongue for 10 seconds, so using an oral thermometer makes sense. In 1970, you might question whether an eight year old could do the same for 4 minutes and it might be safer to just go with a rectal reading. The Bates guide to examination of the time even suggested rectal readings for all pediatric patients.
You could make similar arguments for adult hospitalized patients where it wasn’t that much harder to take a rectal temperature. When oral temperatures were harder to get right, hospital were willing to go rectal for adults in more circumstances than today’s technology would necessitate.
The newer technology gives us more options, so we choose rectal temperatures less often.
Next you have time. In most circumstances, rectal temperatures take longer to get than an oral temperature. Ear thermometers are a little faster than oral (if less reliable).And the newer forehead and non-contact sensors are even faster. (Although they are trash in my opinion. I had one read 98 point something while my plain old digital oral thermometer read 102.)
Since time is money, we choose the faster options even when they are not as good, so we take rectal temperatures less often.
Third, you have prudishness. Around things medical, we seem to have gotten more prudish in my time. I am not sure how much this has been the younger doctors and nurses being more prudish and how much it has been the population getting more prudish and the doctors and nurses just responding to that. ( I really need to make a whole post on this topic.)
We have become more prudish in some ways and don’t want to touch or have reveals anything as icky as a bottom or anus, so we take rectal temperatures less often.
Finally, we take children’s feeling more seriously than we did in the past. This is a good thing overall, but it does make us more reluctant to subject them to things they might find embarrassing, even when maybe we should be prioritizing health.
And again, this means that we take rectal temperatures less often.
All of these factors have contributed to the demise of rectal temperatures. Technology has changes the pros and cons and we seem to be choosing speed, cost, and less embarrassment over reliability.
I don’t know if they will completely go away in my lifetime. I think that there are corner cases like heat illness where they will remain. They might or they might even have a bit of a revival, but they will never be used at the frequently they were used pre-1980.
For what it is worth from an non-fetish perspective, I do wish that we would get rid of the infrared based thermometers and go back to mostly using oral and rectal digital thermometers. I think that if you care what a temperature is, oral digital thermometers are really the best choice 95% of time or so, with digital rectal thermometers taking up most of the other 5%. However, I fear that we are heading to times where even the oral thermometers get used less and less.