No. Clients do not have to be opted-in to receive a Mass Message, although it is strongly recommended in order to comply with texting laws. Sending Mass Messages to clients who have not opted-in increases your risk of being flagged as spam.
If you would like to ignore opt-ins when sending a Mass Message you can do so by following the steps below:
When configuring the Mass Message, there is a toggle switch labeled, "Require client opt-in for marketing messages." Make sure that the toggle switch is turned off and clients will not be required to opt in.
Please note that clients who have previously opted out of messaging will not receive any mass message or individual message unless they opt back in.