Certainly you have heard about the recent DoS problem around the servers, when everyone's ping spikes up to 999. The amount of discussions regarding this topic in L4D2 forum and the lack of help/awareness is concerning.
There's a kind of "solution", that should work for some people, that requires you to change your username to include ~ ~.
For example, my username currently is zzz, and my new username should be zzz~ ~.
I've tested it with some users and it apparently seems to have worked for most of them.
For more information about this, I strongly suggest you to take a look at https://www.reddit.com/r/l4d2/comments/1q8kms/comment/cdaiwyj and at r/l4d2, as this "solution" was taken from there.

Huge credits to u/3yebex AKA Xbye for the discovery and all the information given about the topic and time spent trying to help the community, you're appreciated ❤️

I also would like to highlight how trash Steam moderation is, banning users and deleting their content when they're trying to help. Certainly more people would have been informed about this if it wasn't for the Steam moderators.

If it doesn't work for you, don't complain to u/3yebex nor to those who are trying to help. It's no one's fault that this situation is happening other than the users causing this situation lately. If you can't appreciate any attempt to help the community without criticizing it, then stay quiet and play singleplayer or another game, you will be doing a huge favor for everyone. We are just trying to help. 🙂

Can't wait to get complaints from people who can't read specific words such as "most", "some", "should" and say "stop trying to cause fear in the community, you're not helping anyone!!" and complaining about those who actually try to help the community while they're sitting on a chair the whole day contributing to nothing at all.
Don't come up with "you're not spreading awareness, you're just causing fear" type of messages, the persons getting affected by this wouldn't say the same about it and would, as expected, be grateful with any type of help. Anyone would.
For that kind of persons, ignorance is a choice.
For those who actually didn't realize "most", "some", "should" and peacefully "complained" about it, mistakes happen. You are loved and appreciated.