The tick is a verification badge, which signifies that Twitter has proven the identity of the person holding the account. It is a valued symbol for people who use Twitter a lot and who are afraid they might be impersonated by bogus Twitter accounts.
It is not clear why Twitter de-verified Yiannopoulos, but BuzzFeed writer Alex Kantrowitz has a fantastic account of the controversy here.
Milo - and he's universally referred to as "Milo" rather than by his surname, a bit like Cher - is an egomaniacal, self-styled libertarian provocateur whose opinions may be as much an ironic performance as they are sincerely held. I've talked to him before, and I find him both reprehensible and funny.
Here is a typical Milo tweet:
Easiest way to get banned on Twitter: criticise or ridicule feminism or Black Lives Matter. This is political. @LouiseMensch
- Milo Yiannopoulos (@Nero) January 9, 2016
However, Milo has 139,000 followers on Twitter, who do know who he is. And over the last several months has become one of the leading voices of #Gamergate, and of the increasingly vocal movement of straight white men who believe that they are an oppressed minority in a society dominated by feminists and social justice warriors (SJW's) committed to ending free speech.
For Milo's followers, Twitter's de-verification is proof that the leftists in San Francisco who run Twitter do indeed want Milo to curb his voice. It's a shot across the bows. Liberal media conspiracy blah blah blah.
For his critics, it's a sign that Twitter might - just might - be about to take online harassment seriously. For years, women on Twitter have complained that if they publish anything feminist, or critical of sexism, they are instantly targeted with rape- and death-threats by a semi-anonymous army of trolls, teenagers, and other idiots. Former CEO Dick Costolo once admitted that hostility and abuse on Twitter was costing the company users.
For instance, Feminist Frequency blogger (and video game critic) Anita Sarkeesian last summer used Twitter to complain that she had "lost count" of the number of times men had sent her photos of themselves ejaculating onto her image. Those tweets then spawned this long, detailed, and disturbing set of tweets from complete strangers who, for some strange reason, felt she was being unreasonable by complaining about it.
It's not clear what the specific trigger was for Milo's demerit. The exact series of events is confusing, like Twitter itself.
It may have begun a day ago when feminist writer Holly Wood published a column on Medium describing the abuse she got for criticizing Silicon Valley investor Paul Graham. Milo wrote a story titled "Social justice warrior knives out for startup guru Paul Graham," in which he quoted Wood, and that story appears to have been the inspiration for the messages Wood later received at her Twitter account@girlziplocked, like this one:
Wood, among others, began tweeting at Twitter staffers asking its community support team to do something about people who use their Twitter accounts to send abuse. At some point, Milo tweeted this:
You deserve to be harassed you social justice loser https://t.co/iUmMwMVVAT
- Milo Yiannopoulos (@Nero) December 20, 2015
And then two Twitter employees noticed, and one tagged @Support, which is where you report complaints about people's behavior on Twitter:
@Jason not speaking for the company as I have no idea, but saw this today: https://t.co/WTMotlv6Bo
- Nathan Hubbard (@NathanCHubbard) January 9, 2016
@catrincooper @Support @jack reporting him now
- Michael Margolis (@yipe) January 8, 2016