He’s been raising his kid in Durotar. The player meets him there in 5.1 when Vol’jin sends you to find him. Thrall even specifically states that he needs to confront Garrosh on this matter at the end of the quest line, but he hasn’t until now.
^
my guess is that they overused Thrall last xpac, a couple blues even said as much, and so they tried to tune is down a bit.
However, they neglected to use him as a character when it would actually be fucking relevant and important for him to be there
Yeah I’m not even blaming Thrall, I just wish they had thrown us a bone and had Thrall say something like “I’ve been talking with Garrosh and wwhile I don’t agree with what he’s been doing, he’s the Warchief, and I must make some weighty decisions before confronting him directly.” But nope he’s just going to charge in like some big dumb orc.