The site doesn’t claim that slack.com, twitter.com, etc. will go down when the flag is flipped. Just try entering those domains into the testing widget within the “Domain owners” section, and you can see that their status is not so dire:
> Minor problems detected!
> This domain is going to work after the 2019 DNS flag day BUT it does not support the latest DNS standards. As a consequence this domain cannot support the latest security features and might be an easier target for network attackers than necessary, and might face other issues later on. We recommend your domain administrator to fix issues listed in the following
technical report [link to report]
As other comments have said, the new rule does not require that all domains support EDNS, merely that all domains respond with “EDNS not supported” when applicable instead of pretending that they support it. Those sites you linked follow that rule.
> Minor problems detected!
> This domain is going to work after the 2019 DNS flag day BUT it does not support the latest DNS standards. As a consequence this domain cannot support the latest security features and might be an easier target for network attackers than necessary, and might face other issues later on. We recommend your domain administrator to fix issues listed in the following technical report [link to report]
As other comments have said, the new rule does not require that all domains support EDNS, merely that all domains respond with “EDNS not supported” when applicable instead of pretending that they support it. Those sites you linked follow that rule.