If using "@" (at sign) in URLs like
http://www.example.com/@john/post/10, is SEO affected negatively?
While there isn't a hard and fast rule saying not to use the
@ symbol Google's John Mueller has said in a Google Webmaster Help topic in the past that you should avoid using special characters in your URL's. He specifically makes mention of avoiding commas, semicolons, colons, spaces, quotes, etc, but in general terms talks about avoiding any special characters as URL's with special characters are often harder to automatically link and are harder to be correctly recognised when parsing text content to try and find new URL's.