Why not able to enter Support Channel in setup page which is less than 5 or more than 10 characters

In Slack setup, Support Channel field is been validated for 'Please enter between 5 and 10 characters (can contain alphabets, numbers, hyphen and underscores), starting with an alphabet'. User can enter as per this validation. If user wants to enter previously created slack channel which doesn't fall under this criteria, it will not be possible.