I just tested this (in 63.0.3239.52) and I see the important sites dialog with the following steps:

1. In a fresh profile
2. Visit and bookmark some site
3. Delete cookies from Clear Browsing Data

Are you only deleting Browsing History? That doesn't trigger the important sites dialog.

Couldn't reproduce the issue and the important sites dialog is not planned to launch yet, so closing this.

Bug:
After i Enable "Important sites options in clear browsing data dialog" it worked. But 1 or 2 days later, the dialog dosen't pop up. I deleted my Chrome User profil. No changes. I disabled the function and than enabled it, no changes. The dialog dosen't pop up. I installed a new chrome version, no changes. What make i wrong? I use Windows 7.

The Function:
When i'm always logged in chrome with my google user Account, i don't won't that the google cookies are deleted, when i clear browser history. I have no options in chrome to say, please don't delete google cookies. 

Second the function "Important sites options in clear browsing data dialog" makes no sense, because, i must do the same thing every time again. Why not implement an static whitelist / function, where you can say "google.com" exclude from cookie delete. A static lists of sites like you implement it for Javascript Blocking.

Hi,
thanks for adding more details. It is great to get some feedback about this feature.
It is still being developed and we are actually working on a way to let the user define a list of sites that should be whitelisted from deletion as the current approach of using heuristics is not reliable.

chrome://flags is only supposed to be used to try out experiments and it is expected that some experiments have bugs or don't work at all. 

I will leave this bug open until we decided on an approach and implemented the changes.

Hi Dullweber, great. 

People may expect not to be logged out, after cookie clearing, when they always logged in with their google user account. For me personal it is an inconsistency. The reason why i added my user account to chrome is, i want always logged in. Like in the mobile chrome browser. 

So that was the reason to use the "Important sites options".

We decided that we won't continue development of this feature any time soon and removed the code to improve maintenance.