Squid remains to be the only one that can deal with SSL proxying(yes it's kind of MITM, but it's needed sometimes), and it's also the real "pure" open source. HAproxy might be better fit for enterprises that need support?
Squid is a caching HTTP proxy, which began with forward proxying but also supports reverse proxying. I wouldn't regard it as relevant to modern, dynamic architectures as HAProxy or Varnish (another caching-focused project).
There's no real difference in open source purity between any of these projects, unless you dislike the stewardship of a company. HAProxy has existed for a long time without such stewardship (as has Varnish). Indeed, Squid's lack of commercial backing might be a hint as to its current relevance.
on the other side, you have varnish plus and nginx plus which are closed source, which means their clients can't have access to the source code, they don't know what they run.
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