Being opted-out of a keyword versus not subscribed to a keyword are two entirely different things. If the user has used the STOP keyword to opt-out of
KEYWORD they will not receive any SMS texts from that shortcode. They will need to double-opt into
If you're sending to subscribers who have already opt-ed out, they have an unsubscribed/opted-out status and you can't send them SMS texts until they double-opt back in.
If you're sending to subscribers who have not subscribed, you must have their permission to send that SMS text to confirm if they want to subscribe.
Also you didn't specify if you were sending to a data extension as the entry source of the journey. You also have to make sure that the phone field is in that data extension with the correct datatype.
Subscribe all contacts to a keyword will automatically add the contacts to the All Contacts list in Mobile Connect.
The contacts with Mobile Numbers that enter journey will never get sent an SMS text unless the Mobile number already exists in All Contacts in Mobile Connect via the method above or imported to All Contacts by file import.
But as mentioned above, if they are opted-out, they will not receive any SMS texts.
Mobile Connect documentation: https://help.salesforce.com/articleView?id=mc_moc_mobileconnect.htm&type=5
CTIA Best Practices: https://www.ctia.org/positions/documents/ctia-messaging-principles-and-best-practices