I am not sure what the cause is, but an extraordinarily large number of personal sites seem to have been blocked by Telstra in Sydney, for no reason I can find.  For example, this site: https://denislearns.tech/building-python-asyncio is blocked by Telstra with a scary warning like below:

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But there is nothing malicious about that website. The archive link is https://archive.is/CNtqc which shows a very reasonable personal blog.