I don't really get how walker herds can even still be a thing anymore. Seems to me like a zombie, especially one that's been dead for years from the initial outbreak (which would be the majority of those herds, I'd think) should be too badly decomposed by now to be a threat. Sure, people still become walkers if they die and don't get put down properly, but that should only account for a walker wandering around here and there, not massive herds. But we are talking about a fictional zombie apocalypse show, so I probably shouldn't get too deep into that sort of thinking.
And yeah that "the walkers are evolving!" line totally made me think Whisperers. If I remember correctly, the Whisperers showed up in the comic not too long after Negan's downfall. So if you consider the time jump that happened in the show, it's probably about time for them.
Not a bad way for Rick to go. So basically, looks like he's just MIA and not dead. It leaves the door open to hype the shit out of "RICK'S RETURN!!!111111oneone" if Andrew Lincoln ever decides to come back to the show. And I'm sure Norman Reedus is probably enjoying that little payraise he got. :P