I've seen Method use Demo locks in Mannoroths kill video and both demonology warlocks were demonology.
Is demonology still a viable spec?
Once you have the trinket from archimonde, Demo is perfectly viable for anything in Norm & Hc. Works for some bosses on Mythic (i'm only 4/13 but I did demo on council eg.)
made by Shyama
To contradict my earlier statement, Demo is actually the least dependent on obtaining the 4Pc (T18) and class trinket, although these do provide substantial benefits. I don't know about using Mythic T17 4Pc, especially since now people are probably getting Mythic T18 4Pc, but like Octa mentioned T17 4Pc has been used on fights like Xhul'orac.
The core strength of Demo is still the same:
1. Extremely strong execute with Serv/Serv.
2. Largely unaffected by movement regardless of talent choice.
3. Highest damage potential out of just about any spec in the game within a 10 second window with Demonbolt.
4. Arguably gains the most single target damage out of all the Warlock specs when there are cleave targets present.
Service/Demonbolt will also be extremely effective once you get your ring as it has extremely high burst potential for every ring use.