How often? Checking my inguinal (groin) temperature happened to me almost every day when I was hospitalized for nearly a year with rheumatic chorea in the late 1950's. It was the safest and most convenient place to put the thermometer because of the tremors and unpredictable movements my body suffered.
I'm far from being an expert, but from what I've read, where (vaginal, rectal, etc.) the temperature is taken is immaterial as long as it is taken in the same place and time of day (preferably in the morning immediately after waking up) every time because basal (when core temp is at it's lowest during the day, usually upon waking) temperature rises only after ovulation, so does not predict it. Same time and place is critical for checking ovulation because the change will be in the range of ½ to 1 degree (F).
When a woman checks her temperature without assistance, checking in the vagina is, probably, much more convenient than rectally, and is just as accurate as checking temperature any place else. Because of proximity to ovaries, vaginal checks are probably the most precise (I don't know, but a gynecologist might) as long as the checks are the same time every day. https://pubmed.ncbi.nlm.nih.gov/2928151/
Taking inguinal (groin) temperature may, or may not, be a common medical practice today, but it is real one, and just as informative as taking oral, rectal, vaginal, or axilla temperatures. See link at end of paragraph just above this one.
Maybe we shouldn't be dismissive of what we see or read when we are only opining. 😊