Is a URL allowed to contain a space?

Question

Is a URI (specifically an HTTP URL) allowed to contain one or more space characters? If a URL must be encoded, is + just a commonly followed convention

Answer 1

Can someone point to an RFC indicating that a URL with a space must be encoded?

URIs, and thus URLs, are defined in RFC 3986.

If you look at the grammar defined over there you will eventually note that a space character never can be part of a syntactically legal URL, thus the term "URL with a space" is a contradiction in itself.