Some web pages don't send referrers by making all links rel="noreferrer". Mastodon used to do this by default, though now they've changed their stance.
Links opened from non-browser apps don't have any referrer information either. E.g. if somebody shares your link on iMessage, WhatsApp, or Telegram.
Email clients may also strip out referrers, but I'm not entirely sure about this one.
If people read your work via RSS readers, you'll almost certainly not get any referrers. Unless it's a web-based reader like Feedly.
My website gets a lot of traffic marked as "Direct / None" by Plausible. I suspect this is traffic from RSS readers or Mastodon, but I can't be sure. A few times I've considered adding a "?ref=RSS" to all URLs served to RSS readers and "?ref=Mastodon" to everything I post on Mastodon. But like the author of this post, I feel uncomfortable tracking my readers like this.