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Could be many things, having a game revoked for example. It will say more if you.. click it and read.

could have at least included a screenshot or smth

Most likely a game was revoked. They should read it to find out more about it.

I you click on the yellow bar, it'll show you the reason, most likely a game getting revoked.

It's common to receive the alert when you activate a beta key and after a while it gets revoked.

If it looks like this then it is legit.

View attached image.

Yellow is warning alert, most likely games being revoked from your account, likely beta passes and other cases like that. Not that common, but common enough to scare people from time to time.

Red is serious stuff, trade bans, community bans, account suspensions, VAC bans and all other unpleasant situations. If you ever see that one you can start to panic.