Kirby's dair:
If we shield it, it is -10 on shield. Jump-canceled UpB OOS is optimal but if you don't trust yourself, keep shielding or roll depending on where Kirby is facing. Here's Stryker's input: "Because of the landing hitbox, it will be -10 OOS. That means (in theory) Up-B, bomb (Kirby though, so bomb may go over or through him if he is pushing in), grab (unless they space it on the edge of our shield) and USmash OOS can punish. USmash and grab need to be frame perfect (or buffered), the other have 2 frames of leniency. Up-B can always punish, but you need to be quick. I don't know how realistic it is. Nimious says to add two frames when looking at OOS punishes, so Up-B may work."
Avoiding it in the first place: Don't throw out random moves. Kirby can and will bait you to commit by floating above you and airdodging. If he's high enough you can try an upair (it lasts through the airdodge) but if he's low, you can try a risky uptilt. Personally I don't like being directly under Kirby, and rolling away is an option. So is dash back pivot ftilt, but they might catch you dashing if they go for the dair. Really, we should just try to reset neutral so Kirby has to approach again in the first place; we can impede Kirby with bombs, rangs, fairs, and zairs (retreating fairs and nairs are often good).
Thanks to Stryker, we have some data on Kirby's DAir and how to avoid the setup into Dsmash. Some of this data wasn't on Kurogane Hammer for some reason.
"Upon landing there is a landing hit box that doesn't care if the first 2-6 hitboxes connect, it will always come out and has a set kb and has 13-14 frames of landing lag (kurogane doesn't have that for some reason).
1. If they press into you, you can sdi (or even just di) into them and you can come out the other side and completely avoid the landing hitbox, giving you time to do whatever. (The following is that they don't push to hard into you.)
2. SDI away if they are getting most/all hits of DAir. There are times (depending on how far deep they get into you) you can pop out (especially at high %s) and the landing hit will miss and so will DSmash.
3. If only the first hit of DAir connects, the angle is too high and the landing hit may not connect. Not something to look for, just means you get even more time to avoid.
4.If the landing hit does connect, it has set kb so your % doesn't matter. Without rage on Kirby you can DI down and shield. With rage shield does not always work but usually does (depends on where vertically the landing hitbox connects). Without rage you can sometimes DI up and jump (depends on where vertically the landing hitbox connects). With rage you can DI up and jump. So take your pick, both can work at all %s of both characters with and without rage, but down and shield seems to work best without rage and up with jump at high. Don't bother with airdodge, it's more risky to try to get the airdodge to line up with the DSmash and not get stuck in airdodge landing lag.
5. Although the percents of Link don't matter for the landing hit, they do matter for the previous ones as they can change his vertical position. I highly suggest DI/SDI up and away at Link's higher %s if you are getting hit by multiple hits of DAir as it can lead to you being higher vertically for the landing hit and then being able to DI up still and jump to avoid the DSmash or dash grab."
So it looks like you want to SDI up and away if we're at kill %, and jump away. If they're pressing into us, (S)DI in and you should pop out the other side. One more thing from Stryker:
"If you want you can always buffer the jump and then hold shield after the landing hitbox. If buffered right, you will jump and then airdodge if able. If you can't jump and land instead, then you will shield and avoid a DSmash. But I don't know how well people can buffer."