Current rules are:
- 0.35% of very-established users ignore you: no public highlight, but a range on your posts that only you can see
- 0.5% of very-established users ignore you: light highlight
- 1% of very-established users ignore you: medium highlight
- 1.5% of very-established users ignore you: dark highlight

You're very established if:
- You have 100+ hours of online time
- You have 100+ posts
- You registered 30+ days ago

There are ~1414 very-established users right now.

The change only shows up in the default theme.