If you want to open a telnet:// link (for example on an Eve-NG lab) and you encounter the error MacOS: Launch Local Application Telnet Error – Could not create a new process and open a pseudo-tty, the easiest way is to follow this link which consist in:

# Disable Apple SIP (need admin rights / password) and reboot
csrutil disable 

# Restart your Mac, after apple logo appears, press the keys CMD+r
reboot

#Mount file systems as below: (Note: After the reboot, you system will go back to read-write.) 
sudo mount -uw / 

#Get current installed telnet application PATH, example: /usr/local/bin/telnet if you installed in standard Homebrew path
which telnet 

#Create symlink to telnet PATH 
sudo ln -s /usr/local/bin/telnet /usr/bin/ 
reboot

# Reactivate Apple SIP
csrutil enable 

However if you cannot follow this (Enterprise Managed MacOS for example), you can use iTerm2 as telnet link handler and modify the binary used:

1. Configure a telnet profile
2. Change the command use with the right telnet binary and use the magic variable HOST and PORT to pass the parameters
3. Change the URL schemes by choosing telnet

And you’re done ✅️.

PS: If you need to modify a link handler for another protocol, take a look at https://github.com/Lord-Kamina/SwiftDefaultApps