I checked. I'm not surprised if referees of a certain age have never seen it...
"Advice to referees...
See that the player taking the throw-in
really uses both hands; some players are apt to throw the ball with one hand only, using the other simply as a guide"
(in the 1963 Referees' Chart!)
I can't trace an official source to say you can't just drop the ball - but dropping is not throwing, is it? Oddly, toward the end of City v Newcastle, with City killing time near the Newcastle corner, the referee penalised City for a foul throw that was perhaps lobbed rather than thrown to a nearby player but the ensuing Newcastle throw looked much the same and wasn't penalised.
I did find this strange (and wrong?) advice in a Bundesliga webpage: "The direction of the soccer throw must be upwards, throwing the ball downwards toward a teammate is not allowed".
https://www.bundesliga.com/en/faq/what-are-the-rules-and-regulations-of-soccer/throw-in-31111