I recently noticed that zsh and fish will instead show a character indicating a missing linefeed, and still start the prompt where you’d expect to find it:

vidarholen-vm2% echo -n "hello zsh"
hello zsh%
vidarholen-vm2%

If you’re disappointed that this is what there’s an entire blog post about, you probably haven’t tried to write a shell. This is one of those problems where the more you know, the harder it seems