Not as much of a thing as it used to be, but yes, it very much still is a thing.
There are no gamebreaking tactics anymore, but there are just very good tactics out there, you can find some incredible ones in tactic testing that can make your team overperform massively.
That said, it's not everything. Your players should fit the tactic at least somewhat, and sooner or later, you will need to bring in better players.
Also, even for good tactics, there might come a time somewhere down the line where you might have to tweak something.
Another note: Say you are a team that should be in a relegation battle but you have plugged in one of the very good tactis and are in the top 3 by december. The opposition will likely approach you more cautiously than it did before, more defensively as well. Often, this doesn't really affect you that much (of course, depending on the tactic), but it also happens regularly that you don't perform nearly as good in the second half of the season as you did in the first half.
This is something I've often seen in my personal tactic testing, for example. I might have gotten Leganes (predicted 17th) to 1st with like 2,5 points per game by december, but I then averaged around 1 point per game in the second half of the season, which caused them to drop massively (and me to change the tactic).
So, yeah - it's not quite as much of a thing long term with the same tactic, but really, if you use one of the very good tactics and it somehow fits you, it will easily act as Plug and Play for 2, 3, 4 seasons.