Extremely Low . Large lists like this often have a failure rate exceeding 90%. Many entries are "dead" (offline) or have been blacklisted by major websites like Google and Amazon.
For those needing proxies for testing or light browsing, consider these more reputable options: Bright Data Download 5260x HTTP Proxies List txt
Very High . Approximately 79% of free proxies do not support HTTPS, meaning your data is transmitted in plain text and can be read or modified by the proxy owner. Extremely Low