GDPR could be a factor since it required shutting down accounts of people aged 13-16 that did not have parental consent. Getting parental consent required having parents sign a form and send it in.
That's not true. Consent is only one basis for processing. If they stick to using it for legitimate interest reasons then they don't need consent. Even ads can be a legitimate interest, and probably are in twitter's case, though the user is always allowed to opt out of that. There's also nothing that requires parental consent to involve signing a form.