The 1.5.5 version of Preproxy for Mac was available to download on the developer's website when we last checked. We cannot confirm if there is a free download of this app available. According to the results of the Google Safe Browsing check, the developer's site is safe. Despite this, we recommend checking the downloaded files with any free antivirus software. The application is included in Internet & Network Tools.
From the developer:
Preproxy acts as an intermediate proxy server that removes the need for local programs to authorize at the remote proxy server. Preproxy is basically an improved, sand-boxed, Mac App Store-enabled version of Authoxy. It was made because Authoxy has been abandoned for quite some time and has started having problems since macOS Sierra came out.
You may want to check out more Mac applications, such as NetCheck, QuickWho or Super Ping Pro, which might be related to Preproxy.