An HTTP anchor itself is not "unsafe content". And Google will not penalise you for this, as the following article states, quoting Google's John Mueller (24-Feb-2020):
Q: Can it be problematic if our https site links out to non-https external sources in our content (or they were http at the time but 301'd to https eventually, which we didn't update on our end?)
John Mueller: No, that's fine from an SEO point of view. From a security POV linking directly to the HTTPS version when it exists is better, but sometimes HSTS helps. For embedding (images, iframes), you need to use HTTPS.
Yes, in the interests of your users, you should be linking to HTTPS where possible, but this is not an SEO issue.
Insecure/unsafe content, on the other hand, is when you embed images or other scripts/external resources directly in your site over HTTP (mentioned in the last sentence of John Mueller's Tweet above). As in the first reference you link to. You have little choice in this matter as the browser actively warns the user and refuses to load such content.
(The second reference to Quora is a bit of a garbled mess of opinion - several of the answers seem to have missed the point of the question. However, the highest voted answer is hitting the mark: "If you have links to other sites or internal pages that are http, that shouldn't affect anything.")
And if a URL is posted as plain text; that's just plain text.