Thank you for reaching out to Atlassian Community!
The ability to restrict comment in Jira Cloud, currently, is available only for Classic projects.
Testing here and also checking the feature requests, this option was added recently for the new issue view, but not for next-gen. Based on that, I created a feature request suggesting to add this option to next-gen projects as well:
Please, click on vote and watch to receive updates about the feature.
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