What is the __Host- prefix?

The answer

__Host- is a cookie name prefix that makes supporting browsers accept the cookie only if it has the Secure attribute, was set from a secure page, has Path=/ and carries no Domain attribute. MDN states this guarantees the cookie is sent only to the host that set it. The result is a cookie locked to one host that sibling subdomains cannot overwrite. It is a strong default for login sessions that do not need to be shared across subdomains.