Why is there no “Disagree” button any more?
Because there was nothing it did that leaving the box unticked does not. Not ticking it IS declining, the button to create the account stays disabled until you do, and Back is where it always was. A second control worded as a refusal made it look like declining was a step in creating an account rather than the end of one.
From the guide Creating Your Seayora Account.
Confirming your mobile number — whenever you like
Once a number is on your account, the setup checklist offers to text it a code to confirm it. Nothing on Seayora is refused because a number is unconfirmed, and no text is sent unless you ask for one.
It is also in Settings → Security under “Your mobile number”, on the website and in both apps. Confirming it is what lets a sign-in code, or something urgent about your home, reach a handset we know is yours.
If you have replied STOP to a Seayora text in the past, that number cannot be sent a code — Seayora honors the opt-out and says so on the screen rather than pretending the message went.
Read the whole guide: Creating Your Seayora Account