Here is a test that anyone can replicate:

Step 1 - User A: Create an Album Photo.
Step 2 - User A: Upload a Photo to that Album.
Step 3 - User A: Comment in that photo.
Step 4 - User B: Liked the comment of the User A.
Step 5 - User A: Deleted the photo from the Wall (Photo still exist inside in the Photos sections and in the album)
Step 6 - User A: Clic the "Delete All Reactions" button.

The reaction notification is not deleted.

Ok, after investigating it's seems this behavior.

As a Admin, i create a Poll (Premium Component) then i deleted that from the wall (after the users commented and reacted to that) but the poll still exist.

Some user reacted to my comment:

/notification/read/19610?notification=

Then that user comment in the publication :

/notification/read/19612?notification=https%3A%2F%2Fwww.example.com%2Fpolls%2Fview%2F6199%2Fsomepolltitleexample%23comments-item-5210

I confirm that guid (19610 and 19612) are from the poll and from the same owner (user that comment and reacted)

The 19610 is the notification that is not deleted by the button "Delete All Reactions"

Explained Summary:

If the post is deleted from the wall, but the object still exist (like the poll situation) the reactions are not deleted by the button "Delete All Reactions".

i just searched my complete locale files for 'reacted to the comment' to no avail.

Could you please provide the exact message to give me an idea where it comes from.
or better the components name. (maybe it's part of premium ossn?)

I tried to recreate that in a VM, without luck.

Only i found this in my prod environment with my admin account, i have some notifications "x user reacted to the comment" i deleted all the post in my admin account. So I wonder why this notification still in the nofication page list.

the notification url are like this:

https://www.example.com/notification/read/20084?notification=

after the notification= is empty.

Is a orphaned notification, because the post doesn't exist anymore. I can manually delete with the component of Michiel (the ❌ button) but the button "Delete All Reactions" doesnt include that orphaned notification.

Hmm, normally a notification gets deleted automatically if the object it is based on has been deleteted.
Just tried:
A made a new post on the main wall
B commented on that post
A's notifcation counter gets incremented by 1
A opens the top right notication box and clicks the 'B commented on a post' entry
A gets redirected to a new page like post/view/230#comments-item-163 showing his post
A - still on that page - deletes his post
A opens the top right notication box again: the notification has disappeared
A opens his notifcations/all page: the notification has disappeared here, too
Everything as expected.

So please provide your exact workflow to reproduce your issue, Hugo.

Sorry, my bad. It was the hook for other components like polls, pages, content report, that are premium components.

I noticed another detail, if i delete a post, but i still have the notification in the notification page list, then i use the button "Delete All Reactions" doesnt delete orphaned notifications (where the post doesnt exist or was deleted) only can be deleted manually.

Is there a way to include the orphaned reactions notifications?

When you go to that notification, only opens a blank page (no 404 error)

@Hugo - it hooks them. all at once. Arsalan gave me a complete list, and it hooks all of them via the list.
If you find one that for some reason doesn't hook - let me know, and I can add it in.

Updated in my site, and now works normally the names and characters or other strings from the locale translations. I will wait feedback of my users too.

I noticed there's no hook for other notifications like from groups, pokes, mentions, etc. 🤔

Re not working unread:
Yeah, that's a funny case of making things unnecessary complicate. :)

If you are SURE you want to output 'Michieal Coder'
you would
echo 'Michieal Coder';

and not

$name = 'Michieal Coder';
echo "{$name}";

Right?
Same here: You want to set viewed to NULL? So just do it!

UPDATE ossn_notifications SET viewed=NULL

Be aware that mysql expects a VALUE (namely NULL) here, thus viewed='NULL' would be wrong
Your original construct
$nix = NULL; SET viewed={$nix} is passing just an unset variable (=NOTHING) and that's why MySQL bombs out.
So if you really want insist on doing it that way, you need to change it like
$nix = 'NULL'; SET viewed={$nix}

Aside from that: all these fields are defined as BIGINT, thus expecting values in all three cases and no strings. So a more correct syntax would be

 $Notification->statement("UPDATE ossn_notifications SET viewed={$nix} WHERE(owner_guid={$owner_guid} and guid={$guid});");

With the new version in, using Hugo's test name from his comment; I change an existing user's name to kate bishop (c) because I had notifications from them.