We block desirable actions when a user isn't registered for a particular Stack Exchange site. For example if a user wants to "favorite a question" or raise a "rude or abusive" or "in need of moderator intervention" flag one must register for the site first.
If the favoritor's profile is deleted then either the information is saved somewhere, or it is lost. If it is lost then it no longer appears on the user's "Main Stack Exchange - Current User - Favorites" tab - so you lose your bookmarked favorites (badges earned are not lost, but deleted users won't take you past the threshold if they are deleted before the event is counted).
We have flags for various reasons, and proper usage of some of the flags does often require one to be knowledgeable about the site in question but flagging for "rude or abusive" should be universally standard.
Flagging for "in need of moderator intervention" should be allowed for users with a certain amount of Flair (I suggest 1000). Any credit/blame could piggyback on the user's top community (if there's nowhere else for it to go).
I sometimes visit Stack Exchange sites where I'm not registered. There is a reason to visit but I don't plan on visiting regularly nor do I want to register for every site. It's not a lot of trouble but it seems unnecessary and adds to the record keeping burden. It's like requiring Citizenship when all that is required is a Visa, and I'm not an association bonus collector.
We might see an interesting question (or answer, but that's a different feature request) and want to favorite it, but without participation we risk eventual deletion and add noise to the site's user list while we lurk.
As a result attempting to Favorite (using the clearly visible ⭐) results in this blue popup: "Please log in or register to favorite this question."
I understand that my browser has bookmarks, the pros and cons of which are discussed here, but that won't preserve the statistics nor record it conveniently on the Main's list (see above).
Flagging options for rude or abusive content isn't even available, but contrariwise we are allowed to improve (edit), ask, or even answer a question (leading one to ponder (rhetorically): "if there's no support for this how could we be flagged?").
Is it a reasonable request to allow rudeness and abusiveness to be flagged, and for established users to be able to send important moderator intervention flags (and to obtain the benefit, or risk, of helpful/disputed/declined flags recorded somewhere). This request seems inline with the CoC expectations for reporting.