Valve has announced that the iconic Counter-Strike: Global Offensive [official site] map, Dust2 will be removed from active duty and replaced with the Inferno map. Dust2 will still be playable, but it exists as a separate map group for the casual and deathmatch game modes. As a result of this swap, you'll no longer see Dust2 in professional esports tournaments.
As per the official Counter-Strike blog:
In the wake of the ELEAGUE Major (congratulations Astralis!), we’re making a change to the map pool: Inferno has returned to Active Duty, replacing Dust II, and will be featured at the next CS:GO Major. As in the past, the updated Active Duty pool is automatically selected when you enter Matchmaking.*
And in Casual and Deathmatch game modes, it's now a bit easier to play your favorite maps. Map groups have been separated into additional categories: Active Duty, Hostage, Reserves, and Dust II.
*(don't worry, it's just as easy as ever to play Dust II. It's now front and center on the Competitive Matchmaking map selection screen)
Inferno returned after a rework in October 2016 when Valve plonked it into the Reserves map group, asking for feedback on the revamp. You can visit the microsite dedicated to the changes if you want to catch up, but essentially it as about making tweaks, particularly regarding legibility of the map and creating options to open up play in problematic areas. Also the sky is a lot bluer.
The swap doesn't feel like a bad thing as, even though I only dip into CS:GO very occasionally, Dust2 felt kind of as close to a solved problem as you get in an analogue gaming situation. Teams knew the strategies and the counters so it was just about who could execute (pun intended) on them better. Hopefully Inferno will throw a metaphorical cat amongst the, um, digital pigeons...?