How to reclaim ^S in zsh?

This is probably flow-control, which is handled by your terminal and not your shell. Typically control-s stops terminal output, and control-q resumes it. See the note in ":help ctrl-s".

To remove this behavior, you could try these shell commands:

stty stop undef
stty start undef

This may not work, though. If you are using a GUI terminal emulator, try looking for flow control settings within its options.

I shot it down in two steps:

1. Disabling flow control in Konsole:

   • Settings > Manage Profiles... > Edit Profile > Advanced (tab) > uncheck 'Enable Flow Control [...]')

   

2. Tell Zsh about it, too:

   setopt noflowcontrol