I locked in the final late 2013 MacBook Pro battery replacement (right before the deadline), and noticed the top case keyboard is slightly different.
The spring mechanisms are the same, but the plastic has been replaced with the newer plastics on the latest MacBook models. Hopefully they don’t have to resubmit for FCC recertification… Kidding. Mostly.
The largest difference is the color – it’s gray, not black. Very dark gray, but not pitch black like the old keyboard.
Personally, I prefer the older plastic as it was softer. This looks more durable, and probably won’t oil over as easily. But it is rougher on the hands.
For comparison, I present my Late 2015 MacBook Air, which had the same keyboard as the Late 2013 in keycap design:
They were the same, and are now definitely different.
Also finally, the trackpad is more flush than past units. The bottom of the pad is not even a measurable fraction of a millimeter offset. Aside from the line, my finger cannot tell there’s a difference. In the old unit, there was a fraction of a millimeter sinkage around the entire pad.
This means they went back and calibrated even after my last top case replacement in 2018 (the battery was built in mid-2017, but my receipts say I got it in early 2018). They’re applying modern MacBook assembly standards and techniques, to spare parts on a machine 5+ years old.
Unfortunately a lot of people won’t get this now, since the only machines still eligible are the 2014 and 2015 15-inch MacBook Pro’s. I presume since they share a top case, Apple felt the revisions would get some added usefulness.