In the tasks.json configuration file, you can specify a task to be defined as a watch task by stating isWatching: true. I don't believe I see any benefit to explicitly stating that a task is a watch task.
What I believe would be useful is if the watch task defined in tasks.json is actually started automatically by VS Code upon opening the workspace.
From what I can tell, currently you need to explicitly start your watch task task <watch-task-name>.
This is an extremely important feature which shall be implemented as soon as possible.
I'd be careful about security concerns, though: On the first launch of a specific project, VS Code should ask if the user wants to enable automatic watching, and warn for potential security issues.
Duplicate of #17147
Closing as duplicate then.
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This is an extremely important feature which shall be implemented as soon as possible.
I'd be careful about security concerns, though: On the first launch of a specific project, VS Code should ask if the user wants to enable automatic watching, and warn for potential security issues.