HTTPS (SEO angle)
HTTPS encrypts exchanges between the browser and the site. A minor ranking signal since 2014, it has become a prerequisite: browsers mark HTTP connections as not secure, affecting user trust directly.
In practice
The residual SEO issue is migration: every HTTP URL must redirect via 301 to its HTTPS equivalent, with no mixed content or dual accessible versions. An expired certificate turns the site into a warning page — automatic renewal is not a comfort setting but a basic availability safeguard.