That's not quite correct.
It is no longer visible on the website itself, or within the mobile clients. But if you inspect the requests, you can still see things like
source: <a href="http://twitter.com/download/iphone" rel="nofollow">Twitter for iPhone</a>
Yes it was removed the UI, but if you browse Twitter with developer tools open to the Network tab you can still find the posting client in the API response JSON when viewing a Tweet.
sReinwald|11 months ago
areyourllySorry|11 months ago
magnet:?xt=urn:btih:c963982c4ffa264fe76ec5918f83dd775521201b&dn=twitter_users_csv&tr=udp://tracker.openbittorrent.com:80&tr=udp://tracker.opentrackr.org:1337/announce
tech234a|11 months ago