As the aforementioned RFC does not include any reference of encoding spaces as +, I guess using %20 is the way to go today. For example, “%20” is the percent-encoding for the binary octet “00100000” (ABNF: %x20), which in US-ASCII corresponds to the space character (SP).
https://stackoverflow.com/questions/1634271/url-encoding-the-space-character-or-20