This is not opt-in how I understand it. When there is no alternative, or the alternative is not using a service, I'd call it a hard requirement instead.
I like how opt-in is handled by GDPR; e.g.: "Consent must be a specific, freely given, plainly worded, and unambiguous affirmation given by the data subject (...) A data controller may not refuse service to users who decline consent to processing that is not strictly necessary in order to use the service.", source: https://en.wikipedia.org/wiki/General_Data_Protection_Regula...
I like how opt-in is handled by GDPR; e.g.: "Consent must be a specific, freely given, plainly worded, and unambiguous affirmation given by the data subject (...) A data controller may not refuse service to users who decline consent to processing that is not strictly necessary in order to use the service.", source: https://en.wikipedia.org/wiki/General_Data_Protection_Regula...