Notifications are messages for specific individual editors. The notifications which should be kept are the ones which the specific individual editor has not requested to be deleted. No reason has been given for this action whatsoever. What is the motivation here? I want to see a practical reason for why you want to delete other peoples messages. No, the fact that they will be kept indefinitely is not a practical reason. Everything on Wikipedia is kept indefinitely unless it is against site policy or outside the scope. Allowing people to manage their own messages is definitely not outside the scope. We shouldn't have to explain that. You wouldn't be very happy if Outlook or Google mail started to delete your messages. Come on now, don't be deleting peoples messages. You are already auto deleting them. Never delete a message until the person that message is for presses a button which says, "Press this to delete these messages". That is the function you should be considering your next step here.
I'm sorry if I seem condescending but this is a condescending suggestion. Rather than think little of you, this is totally like something I might decide was a good idea myself. You are busy people on this stuff. You don't sit around wondering if you should wonder what to do. You just try to select stuff and do it. However, don't delete the notifications/messages. I've posted this a couple of times and nobody has affirmed it makes sense, so maybe you just aren't quick to respond over here, and I think I detect a tone in my response. Honestly, I've been wondering if recent events haven't been similar to when the whole world went to war, that it affects us in an anonymous internationally collaborative scheme, tensions fray without even thinking about them. I don't mean to be rude. Also, I've never done a single thing to help the development of this software. I am certainly not the boss around here or anything totally stupid, because I do feel like I am demanding on this suggestion... a sense of urgency which was probably not necessary.... but all the same, please don't delete peoples notifications. Instead, please add buttons and features that allow them to much easier manage the deletions themselves... Slightly more difficult yes, but ultimately more functional...