I would disagree, You can make Text-only links look fairly harmless too. The issue is more often that people do not look at all, even in text-mail, what the link points to, not that they can't see it (plus you can hover and the browser still shows the URL once you open it)
>Privacy invasion and tracking
Only if you use an ancient and outdated client that doesn't block external resources by default or proxies them over another server.
>Mail client vulnerabilities
Only an issue if the client rolls their own HTML renderer. Outlook uses the IE Trident Engine, Thunderbird uses Firefox' engine, Gmail transforms using the Chromium engine internally. That means the email client is usually as secure as the respective browser, unless you use a mail client that rolls their own engine.
>HTML emails are less accessible
Possibly, depends on the sender. You can make them more accessible than Text though because you can prevent the screenreader from trying to read out the link or irrelevant information.
>Some clients can't display HTML emails at all
Most mail clients will usually bundle a text-version of the mail, some MTAs also do this on their own. Doesn't really matter in practise since 99.99% of people use a mail client that can display HTML.
It feels a bit like complaining that people on the highway drive so fast that you can't properly merge into the rightmost lane with your oldtimer car.
>Rich text isn't that great, anyway
And tbh, this last one basically seals it; this is a website made to rant from the personal perspective of someone who blocks mails containing any HTML from their services and has quite the history of throwing people using their lib under the bus over personal preferences.
I don't see how any of this is relevant when even the site itself admits that using a multipart mail with both text and HTML is sufficient, why should I even begin to send plaintext instead of doing that?
Unless your mailclient is garbage, it handles multipart and will understand the mail.