After today’s update, all passwords, etc cookies got deleted in every web browser. KDE Plasma install.
An update shouldn’t be able to do that.
Did you install any cleaning/privacy/security software?
Well no.
If everything else is working fine and only your browsers were cleaned up it must be some piece of software doing that.
The package update process wouldn’t touch files in your home directory.
Where are the passwords, kept in the home folder? I thought it might be something to do with Kwallet manager.
Depending how your browsers are configured, passwords could be stored in kwallet but I don’t think cookies are stored there.
The browsers store the passwords, I suppose the cookies too. Usually, if one browser is missing a password, I look in the other. I have 6 browsers, and everyone got cleaned up. That’s why I thought it must be something to do with a system app.
I had to go to another distro to find few of the passwords, that I couldn’t remember.
If all 6 of your browsers had the cookies removed, there must be some application doing that. They are stored in different places for each browser.
Which packages were updated? Check /var/log/pacman.log
Which browsers, specifically?
Another possibility is broken permissions (e.g. browsers were run using sudo), or profile-sync-daemon broke.
The pacman.log is terribly long.
Browsers: Firefox, Chromium, MS Edge, Opera, Vivaldi, Yandex
Sounds like someone used bleachbit or stacer. Does anyone else have access to your machine. Sweeper in kde may also be a culprit.
No one else.
Yes it is. But look at the part for today’s updates 
I will, thanks. I actually mentioned this as information of what happened. I won’t lose the passwords, as they are in other 2 devices.
This is indeed an intriguing “accident”. So perhaps it’s good for the sake of “research” to know what exactly happened.
Here’s today’s pacman .log Copied everything with the date 2021-02-15
Looks pretty normal other than the fact you are using pacman -Suy to update and you have a .pacnew file that should be attended to.
Is something wrong with that?
No not wrong just unusual.
Actually, I am using $ yay -Syu