Here goes my attempt at putting this thread out of its misery:
Regarding the original post, the functionality already exists. The reading status of announcements is saved in your browser's Local Storage. Items saved in Local Storage do not have an expiration date and should persist until you manually delete them.
If you are regularly clearing Local Storage and want the setting to be saved server-side, the answer will be the same as in other similar threads. Some settings will be moved to the server in the future. You likely won't get an ETA or confirmation that this is one of those settings.
Your posts on network security and cookies, I'm afraid, read like the ramblings of a luddite.
Your
first reply conflates your requested convenience feature with matters of web security. Any website that provides a customized experience for its users needs to store information locally. For a website with a login this could be as small as a random session identifier. It doesn't really matter where this information is stored (Local Storage, Session Storage, Cookies) but if you delete this information the website will forget about you. It is also common to save some additional information locally for various reasons. As an example, MangaDex stores reader settings locally on purpose because people will have different settings depending on the device type. I do actually agree with you that it would be nice to store dismissals of announcements server-side. But that is a matter of convenience not security.
In your
next reply you conflate the concepts of privacy and security. You link an article that talks about privacy concerns with cookies. Next you link to MangaDex's previous data breach which was caused by an RCE and had little to do with "cookie security". Having "to re agree" the use of cookies in any new browser you use is an inevitability of all websites with login and account functionality. Most people are happy with the "overuse" of cookies (Local Storage) because they do not sign into new devices very often and the announcement shows up only once at the beginning for any web browser in its default setup.
Your
third reply already contains mostly incoherent ramblings about some cookie going rogue and stealing your banking information. This, again, confuses the privacy issues with cookies for security issues. Cookies are often used to "steal" marketing-relevant information about you. The companies behind these services are trying to gather as much information about you as they can so that they can place more relevant ads and get you to buy more unnecessary things. They are not trying to find out your mother's maiden name to social engineer your bank into transfering all your money to them. The specific purpose of tools like uBlock Origin is to block these types of cookies without disrupting site functionality. Which is why MangaDex works just fine with it enabled. Blocking all first-party cookies will break pretty much all websites with any type of login.
This reply encapsulates your tech illiteracy very well. You claim that telling the average user to enable cookies is "terrible advice", but all mainstream browsers (even more privacy-focused ones) enable at least some cookies by default. They do this because blocking all cookies breaks, within a margin of error, all websites on the internet. You then again continue with the whole "stealing bank logins and passwords" claim. Please provide a source for a single hack where banking logins where exfiltrated using cookies from an unrelated site.
What do you think "local" means? It's called Local Storage, because it is local to the specific web browser.
Generally, you are preventing the MangaDex website from using legitimate tools such as a persistent Local Storage and complaining about degraded functionality. Your initial request is actually quite reasonable after you explained it a little more clearly and I have upvoted it to show my support. As with any suggestion costing dev time, it's a "maybe" on if it will be implemented and a "someday" on when. It does not seem like the highest priority item.
The rest of this thread is, frankly, crazy. Cookies are not like a gun, loaded or otherwise. They are like kitchen knives. You can certainly stab someone with a kitchen knife, but they are an important tool that most people have in their home. You are the one advocating for keeping a single blunt kitchen knife in a lockbox in a cellar, wondering why it's a pain to use your kitchen to make a meal, and acting like we are all weird for just keeping our knives in the kitchen.