⚠️ Before you look at my answer to this I urge you to reconsider your plan. What is your real problem? Is it that you can't filter by looking for at-mentions, or the fact that you have so many notifications that you need to? Can you target the source of these notifications? Adding this rule everywhere will introduce more notifications and might lock you in to the problem. Also, it might make your way of working harder to change and improve. If you're still intent using this solution click the little triangle to expand this section.
How you can go about it would depend on your subscription tier.
At Enterprise(+) you could add the rule to a bundle, and add that bundle to all projects and project templates. You could also add the bundle to all projects in a portfolio at once. More about bundles here: Streamline work using Bundles
If not you could add the rule to all projects and templates, but I’d advise against it, as that is a lot of work, and would build a lot of resistance to change.
Good luck!