I noted in 2020 that “in Britain, ‘pensioner’ might refer to a person who is no longer working but is not necessarily receiving a pension: what Americans would call a ‘retiree.'” However, “American uses of ‘pensioner,’ what few there are, tend to refer specifically to people getting (often particular) pensions.”
Yesterday’s mail brought the first American example I’ve seen of the British meaning:
The pensioner is me but I do not receive a pension from the state of Delaware or my former employer, the University of Delaware. I do receive medical benefits, so it’s not exactly pensioner-in-the-sense of retiree. But it’s close.
notoneoffbritishisms.com 
In the UK, I believe the term “pensioner” applies to someone of state pensionable age, irrespective of whether they qualify for or have chosen to defer receipt of their State Pension. For many years this was at age 60 for women and 65 for men, but has been advancing to 66 for both of late. Their employment state is irrelevant as is receipt of a private or work-related pension. Although one may claim to have retired, ie. stopped working, the term “retiree” does not appear to be that common.
I started receiving a state pension four years ago, but I retired on a work pension nearly 20 years ago, but I’d never call myself a pensioner. I’d say I was retired, but retiree just sounds wrong.
Similarly, I started receiving an Armed Forces pension some years before I stopped work. I retired when I was 64, but only became a “pensioner” when I reached 65. I have never referred to myself as a “retiree”.
An acronym which seems to have vanished is OAP (sometimes spoken in full: old age pensioner). It was coined when the Old Age Pensions Act was passed in 1908, and was still common 50 years ago.
‘Acronym’ is actually a newer word, popularised in Germany (Akronym) in the 1920s, and copied by us a few years later. So if ‘acronym’ passed into American use from the UK, it qualifies as a noob; possibly in a similar category to WW2-catalysed noobs like ‘clobber’ and ‘a piece of cake’.
If there is room for linguistic pedantry here, I will offer this, with an apology:
OAP was always pronounced ‘Oh-Ay-Pee’, so it is an initialism. If we had pronounced it to rhyme with ‘rope’ then it would have been an acronym, as is ‘NOOBS’.
OAP as an acronym has certainly vanished but as an initialism it is used all the time in the UK. Another word sometimes used to descibe us is “twirly”.
BILL051: Might as well add that “twirly” comes from pensioners having free bus passes that can be used after 9:30 am. It was originally bus drivers’ slang, from the common experience of an OAP at a bus stop in the morning, waving their bus pass and asking, “Am I tw’irly? Am I tw’irly?”
OAP seems to have been replaced by “senior citizen” or “senior” over recent years, at least in written form such as on entry price boards.