Who is this user, who needs an FTP client, but only sometimes, and doesn't know where to find one?
Me. I don't want to install a FTP client for the 2 times at year that I go to a site to download something (typically drivers for some obsolete device) and the download is from an FTP server.
And yes, if I have the URL I can use curl to download the file from FTP, even if downloading it from the browser would be easier. But most of the times there is a link to the server directory, with multiple files to select. Yes I can use Windows explorer and connect to the FTP server and browse it, but to be fair, or just open Internet Explorer and past the FTP link.
A lot of sites, especially old ones, are build with the assumption that every browser can access FTP links as you would with HTTP. And so for example a download section is a link to a FTP server.
To me removing it is stupid. Is it a security concern? Not really. Also not having it in the browser will not make security better, a person that needs it will use it with another client. Will make the browser faster or smaller? Not really, a FTP client is something really simpler, and browser have them since ever.