Twitter doesn't do this anymore, probably because they don't want any popup to interrupt users scrolling through more ads, logged in or not. Instagram and Twit

Twitter doesn't do this anymore, probably because they don't want any popup to interrupt users scrolling through more ads, logged in or not. Instagram and Twitter's login prompt might have been motivated by a desire to increase number of logged in users, and Twitter might have given up on that in recent months considering the increased ad load.