Let's Encrypt devs decided to put those short-lived certs (instead of the long-term certs everyone was using) making it almost irrelevant to use that "security header", almost every webdev knows the issues present on the current cache-invalidation problems on every major browser.
Let's Encrypt devs decided to put those short-lived certs (instead of the long-term certs everyone was using) making it almost irrelevant to use that "security header", almost every webdev knows the issues present on the current cache-invalidation problems on every major browser.