https://en.wiktionary.org/wiki/reticent (2)

https://www.merriam-webster.com/grammar/can-reticent-mean-re...

You're correct that some people recommend against it, and it's only been in use since the second world war.

It's also Merriam-Webster, a descriptivist dictionary. Others are of a prescriptionist bent. For example, Cambridge: https://dictionary.cambridge.org/us/dictionary/english/retic...

More on prescriptive vs descriptive: https://twominenglish.com/prescriptivist-vs-descriptivist/

I personally am more descriptivist when it comes to English. It is also a lot of fun to bring some M-W citations in order to "Well, actually" someone who is "Well, actually"-ing.

I only knew of the "reluctant" meaning.

Languages mutate quickly.

Yep, it's what I've heard most often too (I only just learnt the other meaning from the kind person prompting me look up the definition). I also don't think of it as that much of an unusual word, but hey.

Reticent is a valid synonym of reluctant.