In Alpharad's "battle royale" King K. Rool Video, I see multiple occurrences (here's one, and here's a better one) of down-throw (burying the opponent), jump, and up-special for some very early kills (especially on platforms). Is this a true combo?
I suspect that one has to time the jump based on how much the opponent mashes, but I'm not sure.
Best Answer
No. The big reason is that, given you are on the same platform, Up Special's hitbox is too high to hit buried opponents. So you can't hit them until they unbury into the air. In the link you posted, it only comboed after he both unburied and missed with Back Aerial! This was also pre 2.0.0 patch which lowered the Up Special horizontal hitbox.
When they unbury into the air is highly dependent on opponent mashing, which even at 0% can cause at least 17 frames difference in unburying. So you have to call when they enter the air.
You also struggle to position yourself well to hit them with Propeller pack especially since the 2.0.0 patch. You need to jump and lean forward to get close enough to hit them, but doing so gives your opponent time to react e.g. with an aerial attack. Even if the opponent had high damage, giving you time to move next the opponent, you still need to call when they unmash. In those scenarios, it's far easier to hit with other moves like Up Tilt or Side Tilt instead, which have better damage potential, better KO potential, and can be done immediately after using Down Throw. In fact, Down Throw is a true combo with multiple moves at high percents (which are often KO confirms at these percents), but Up Special isn't one of them.
Even if you hit the opponent with Up Special, it's a move that can be DI'ed out of (more so since the 2.0.0 patch). You'll need to call where your opponent will DI in order to keep on hitting them, else they'll escape your Up Special.
In fact: