Why sync servers keep deleted records placeholders??

Those records do not necessarily mean that Firefox is keeping your deleted passwords or credit card details as active data.

Firefox Sync uses server side records to keep different devices in sync. When an item is deleted, the server may need to retain metadata or a tombstone so that the deletion can be propagated to other devices that have not synced yet.

That would also explain why you see different behaviour for bookmarks and other data types. Each Sync collection can handle deletions differently.

I would not interpret the presence of these records alone as evidence of user tracking. The more important question is what information is actually stored in those records and for how long.

If you can post an example of one of the password or credit card records shown by the about:sync extension, with any personal information removed, someone may be able to explain exactly what the server is retaining.

Thanks for the explanation, some issue is I had to do "wipe clients" some times as the pass records there still had credentials, now are more likely:

{

 "id": "{XXXXXXXXXX-ZZZZ-YYYY-XXXX-ZZZZZZZZZZZZ}",
 "deleted": true

}

(those are marked as tombstones)

Weird is that I did that action at the past ("wipe clients" , as the extension explain that is "Wiping an engine removes data from the server. It does not remove local data...." ) till there was nothing on the server (zero records of them)

I read that only solution is to delete my FF account then recreate one would be nice to have a "nuke/purge my server sync data" option on the account.