It depends on the mythic raiding guild. I applied to one and they requested an interview with me. The first question they asked me was how open I was to be multispecing in 9.2. Mage specs for the most part are covenant specific when it comes to performance.
Their expectations contrast the guilds I have raided with in 9.1 where I played whatever I want. I didn’t play fire at the start because it wasn’t competitive so I picked arcane. I don’t like frost gameplay in raiding. I have been playing more frost on my alt though in case I am asked to switch for it on certain fights next tier.