Q: Why proxy scrappers don't include protocol in proxy URL?
A: There are three types of ProxyParser: MixedParser, HTTPParser and
SOCKSParser.
MixedParser is for "special" scrappers that can grab proxies
with both protocols from the same source.
These scrappers are the exception to the rule and should indicate proxy protocol in URL (e.g. socks5://1.2.3.4).
If they don't include protocol, the default protocol will be defined by --proxy-protocol - the only scrapper that does this is the FileReader to allow input proxylists to not include protocol, assuming that they are all the same. In short, if a source has proxies from multiple protocols, the protocol must be in proxy URL.
Since most scrappers are associated with specific protocol, they will be aggregated by HTTPParser and SOCKSParser and proxy protocol is already set by default.
These ProxyParser sub-classes will perform basic duplicate filtering and because of this it's best, performance wise, not to include
protocol in proxy URL while scrapping.
This may be refactored in the future but for now it's stays this way.