Is dropping a weapon “free”, in terms of action cost?

I’ve seen many people on StackExchange and other forums mention that you can drop something as a free action, but I’ve never seen it sourced, or come across support for it in the Player’s Handbook. The "Other Activity on Your Turn" section (p. 190) makes no mention of dropping items.

I’ve personally been ruling that dropping an item counts as an "interaction with the environment", but seeing how frequently I’ve seen people say otherwise I’m assuming I’m missing something.