My iPhone 11 battery health is 84%. Should I replace the battery??

Your decision.

Lower battery health means less battery run-time.

If the current run-time is adequate for your needs and you are planning to replace this iPhone with a newer or new model, say, later this year, maybe not.

If you’re finding run-time inadequate and needing to re-charge during your usage, maybe yes, time to replace the battery.

Replacement cost is USD$69: iPhone Battery Replacement - Official Apple Support (or free, if it hits 80% when you’re under warranty.)

Battery replacement might be a repair or might be a device swap.

the recommendation is to replace the battery when it drops below 80%

You shouldn’t. At least not yet... once it’s 47 - 50% that’s my recommendation. As that’s when I notice faster battery drain.