Who sends you personal messages on LinkedIn? Personally, I don't even trust LinkedIn to have an app installed on my phone. But I haven't received even a single spam notification from any of the major chat platforms.
Interesting. I have all of WhatsApp, Facebook Messenger, Signal and Telegram (+ SMS and email, but I'm guessing these aren't the issue) installed on my phone with notifications enabled, and I only get notified when I receive a message. I'd be interested to know which services are spamming you.
The Google Search/Assistant app seems to be one of the worst that I have installed. No matter how many categories I mute I seem to get a new spam notification every couple of weeks. Unfortunately there are 2-3 useful notification categories so I haven't completely shut it down now. Google Maps also used to be pretty bad for this but I don't seem to recall any new categories for many months now.