You have to get more creative with ROB to get kills now. You can no longer rely on down throw confirms and you have to make use of his full toolkit, including edgeguarding and jablock setups. Let me share everything I know.
0-10%: You aren't going to kill starting at this percentage unless you hit the zero to death. I wouldn't fish for it, because it has to start with a very unsafe and punishable attack on shield.
10-40%: Once you hit 10%, back throw (and forward throw) starts to put opponents into tumble and you can start doing some neat jablock combos. Generally I would back throw, down tilt a couple of times, and then either dash attack and chase offstage with fair and potentially side b, or do jab 2 to try for a second tumble situation if you have enough room on the stage.
40%: Around here, jab starts putting opponents into tumble and you can get jablock setups off of this. N-air also begins to confirm into side b at the ledge for a kill, and side b offstage straight up kills many opponents. This N-air confirm can work for a while on heavier characters; it destroys the likes of Dedede and K Rool.
~55%: F-Air starts to setup for tumble here, and you can soon jablock into a forward smash for the kill. If you want to confirm the f-air, you can do it off of down tilt, which is a fairly safe frame 3 poke.
~90%: D-Tilt begins to chain across the stage here. You can use this to rack up a lot of percentage and get into edgeguard situations, and if you have a grounded gyro you can push them into it to confirm into any smash attack. This is my personal favourite kill confirm.
~120%: D-Tilt sends into tumble. If they don't tech, you can upsmash, and you can techchase upsmash for the kill.
General strategies:
1) Down Throw: This move is generally weak online, but works better in person when you can hear people mashing. At lower percents you're better going for F-Throw -> Dash Attack -> F-Air, U-Throw -> F-Air or B-Throw techchases, but at percents where U-Throw stops comboing you can begin to make use of this. That's not to say the bury is good - it's not - but it puts the fear in a lot of opponents and you can often just react to panic airdodges to confirm smash attacks. Against larger characters, you can up-tilt to catch them (mashing or not) and confirm into up-air on the majority of opponents. Against most, if you know they'll mash you should go for up-air. Generally you should never try to upsmash unless you know they can't or won't mash as they can escape it very early.
2) Ledge traps: Everyone knows about the Gyro-> Up Smash ledgetrap, as well as the spike setup. I've been having a lot of success also confirming gyro into side b, especially as I can move the side b offstage to catch opponents who stay on ledge.
3) Edge-guarding: ROB is godlike at this. Gyro and laser are fantastic snipes, especially fully charged. Side B will delete stocks if you catch them with it, but it's high commitment. Against some opponents like Falcon or Ganondorf, this is incredibly strong. Against others it's more difficult. If your opponent has a teleport recovery you can side B next to the ledge to 2 frame them without too much difficulty if they teleport to ledge (which they often do) - giving the choice of stage spiking or sending them out, depending on how good at teching you think they are. Side B catches any upwards recovery without a hitbox (Inkling, Dedede, Pichu, Olimar, DHD, etc) really well. ROB also has a very strong spike, which will delete predictable recoveries. Fox, Falco, and Wolf really suffer from this, and it kills very early.
Hopefully this helps a little. We lost the option to kill off of a grab for free this game, but it's worth it for all the other buffs we received.
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On the Side B "buff", all they've done is make the hits link better so that certain opponents don't fall out of it as easily. For example, the grounded version now hits Pichu with the finisher, which it didn't before. It's mostly a bug fix if anything.