[drdeca]

My understanding is that while there are certainly changing social and cultural pressures which influence how often people use they in the singular, or at the least, increasing how likely some are to use it (possibly decreasing how likely others are to?),

that using “they” as singular was not at all unheard of. It is quite plausible that the sources I am remembering were somewhat over-representing how common it was (they certainly would have motivation to do so, in relation to the changing social pressures that you mentioned), but my impression is still that for a long while, it was not especially unusual for people to use it without meaning to make (and without being perceived to be making) any kind of statement about gender/sex, though it may have been (idk) the clear consensus that it was “technically incorrect usage”.

Furthermore, my impression (I could be wrong) is that while using a singular “they” to refer to a person whose sex/gender was unknown, was not that strange, it would have been fairly odd for someone to use singular “they” in the case where the speaker does know the sex/gender of the person in question, and that this is something which has changed substantially more of late due to changing social pressures etc. (compared to the use when the gender/sex is unknown).

I should note that I have not looked into this closely, and any parts which I have I could have forgotten, and therefore these are only the impressions I have, which could be wrong, and should be taken with a grain of salt.