I just created an account because of your incredibly detailed post and the very dumb answer by anon.
I stumbled upon this while googling how to move chats from WhatsApp to Signal on an iPhone, and I just want to add that it seems completely impossible on iOS, since — unlike Android — it doesn’t allow for manual backups, and you can’t move chats from iOS to Android at all. The only option is the encrypted transfer from one iOS device to another, which seems impossible to manually inject chats into.
However, the demand for universal backups and imports seems quite high, and Signal appears to be working on it (I’m not allowed to link, but e.g., see the Reddit AMA with Signal’s CEO: The message history backup option “will be available in the near future”), with Apple’s policies apparently slowing them down. Thanks for your post, you saved me a lot of time when it comes to testing all of those deprecated tools.