Strict-transport-security using Nginx

With Nginx strict-transport-security can be added with this line:
add_header strict-transport-security: “max-age=31536000; includeSubDomains”;

If I ever have to pause/remove Cloudflare, would I be able to just add this line since I have a FULL SSL cert installed via Let’s Encrypt?

If so, is it ok to add the header using Nginx and not enable the HSTS option in Cloudflare?