Similar to functionality which allows auto-linking of issues when one is reference in another issues comments, create an automation rule which auto-adds someone as a watcher of an issue when they're tagged in the comments.

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If you’re creating or editing issues, you can query a project’s createmeta or an issue’s editmeta information to view supported fields and operations.

You can retrieve the metadata for both operations by visiting:

• GET /rest/api/3/issue/createmeta?projectKeys=<string>&expand=projects.issuetypes.fields

• GET /rest/api/3/issue/{issueIdOrKey}/editmeta

This JSON returns all fields that can be used in the Additional fields, including their possible operations and values, however, it doesn't not include the watchers field.

It would be usefulll to be able to manage watchers using Automation Advanced field editing.

https://support.atlassian.com/jira-software-cloud/docs/advanced-field-editing-json/