I try to keep the rose-tinted glasses off, but it does kind of feel like that is the direction design has gone in. Whenever I look up a relatively recent boss encounter, it feels like I'm positively buried in mechanic after mechanic after mechanic, while I recall many TBC bosses to be rather a lot simpler. Not all of them were horrifically punishing, though, but the ones that weren't were rightfully viewed as the ones the more casual guilds got up to, and then frequently no further.
But if I think back to TBC, really think back... I'm not sure they had significantly fewer mechanics, we just ended up dealing with the ones that mattered. I mean, I look up the Helya fight in ToV now and am showered in adds and their abilities, and it seems kind of overwhelming. But when it comes down to the actual raid, it won't be. I'm sure Shade of Akama's adds (not that that fight was hard or overwhelming) had abilities as well, but most of the time we didn't really care all that much.
So yeah, maybe things have evolved towards slightly more mechanics, but I actually don't think its as significant as its sometimes made out to be. If there is any change, it's really just that things were, for several expansions, generally a lot less punishing.