Note before reading: this page refers to this hitstun, and not hitlag which Nintendo thought was a brilliant idea to name hitstun.
It's already known that tumble hitstun has been decreased in Ultimate, but there's still no way to exactly determine it. The formula Ruben originally posted ended up being wrong, based on in-game testing from several people, and the real formula hasn't been found ever since, likely because it now seems to correlate to the also-undetermined balloon knockback.
For reference, and to have an idea of how much overall hitstun moves inflict in Ultimate at tumble percents while the formula is not discovered yet, I've made a table for the hitstun caused by Mario's up aerial (which has the same knockback as in Smash 4) on Marth (because he doesn't get the gravity boost) at various percents, and compared it to the hitstun it would cause in Smash 4, starting from where it first causes tumble and ending at the closest percent right before it KOs. The hitstun cancel windows for air dodging and aerials are included as well.
| Target's percent | Knockback | Smash 4 | Ultimate | Difference | ||||
|---|---|---|---|---|---|---|---|---|
| Standard | Airdodge | Aerial | Standard | Airdodge | Aerial | |||
| 45% | 80.853684 | 32 | 31 | -1 | ||||
| 47% | 82.644211 | 33 | 32 | -1 | ||||
| 50% | 85.33 | 34 | 32 | -2 | ||||
| 53% | 88.015789 | 35 | 33 | -2 | ||||
| 56% | 90.701579 | 36 | 33 | -3 | ||||
| 59% | 93.387368 | 37 | 34 | -3 | ||||
| 61% | 95.177895 | 38 | 34 | -4 | ||||
| 64% | 97.863684 | 39 | 34 | -5 | ||||
| 67% | 100.549474 | 40 | 35 | -5 | ||||
| 70% | 103.235263 | 41 | 40 | 41 | 35 | -6 / -5 | ||
| 72% | 105.025789 | 42 | 40 | 42 | 36 | -6 / -4 | ||
| 75% | 107.711579 | 43 | 40 | 43 | 36 | -7 / -4 | ||
| 78% | 110.397368 | 44 | 40 | 44 | 37 | -7 / -3 | ||
| 81% | 113.083158 | 45 | 40 | 45 | 37 | -8 / -3 | ||
| 83% | 114.873684 | 46 | 40 | 45 | 37 | -9 / -3 / -8 | ||
| 86% | 117.559474 | 47 | 40 | 45 | 37 | -10 / -3 / -8 | ||
| 89% | 120.245263 | 48 | 40 | 45 | 38 | -10 / -2 / -7 | ||
| 92% | 122.931053 | 49 | 40 | 45 | 38 | -11 / -2 / -7 | ||
| 95% | 125.616842 | 50 | 40 | 45 | 38 | -12 / -2 / -7 | ||
| 98% | 128.302632 | 51 | 40 | 45 | 39 | -12 / -1 / -6 | ||
| 100% | 130.093158 | 52 | 40 | 45 | 39 | -13 / -1 / -6 | ||
| 103% | 132.778947 | 53 | 40 | 45 | 40 | -13 / 0 / -5 | ||
| 106% | 135.464737 | 54 | 40 | 45 | 40 | -14 / 0 / -5 | ||
| 109% | 138.150526 | 55 | 40 | 45 | 40 | -15 / 0 / -5 | ||
| 112% | 140.836316 | 56 | 40 | 45 | 40 | -16 / 0 / -5 | ||
| 114% | 142.626842 | 57 | 40 | 45 | 41 | 40 | 41 | -16 / 0 / -4 |
| 117% | 145.312632 | 58 | 40 | 47 | 41 | 40 | 41 | -17 / 0 / -6 |
| 120% | 147.998421 | 59 | 40 | 48 | 42 | 40 | 42 | -17 / 0 / -6 |
| 123% | 150.684211 | 60 | 40 | 50 | 42 | 40 | 42 | -18 / 0 / -8 |
| 126% | 153.37 | 61 | 42 | 52 | 42 | 40 | 42 | -19 / -2 / -10 |
| 129% | 156.055789 | 62 | 43 | 53 | 43 | 40 | 43 | -19 / -3 / -10 |
| 131% | 157.846316 | 63 | 44 | 54 | 43 | 40 | 43 | -20 / -4 / -11 |
| 134% | 160.532105 | 64 | 46 | 56 | 43 | 40 | 43 | -21 / -6 / -13 |
| 137% | 163.217895 | 65 | 47 | 57 | 44 | 40 | 44 | -21 / -7 / -13 |
| 139% | 165.008421 | 66 | 49 | 58 | 44 | 40 | 44 | -22 / -9 / -14 |
| 142% | 167.694211 | 67 | 50 | 60 | 44 | 40 | 44 | -23 / -10 / -16 |
| 145% | 170.38 | 68 | 52 | 62 | 45 | 40 | 45 | -23 / -12 / -17 |
| 148% | 173.065789 | 69 | 53 | 63 | 45 | 40 | 45 | -24 / -13 / -18 |
| 151% | 175.751579 | 70 | 55 | 65 | 45 | 40 | 45 | -25 / -15 / -20 |
| 153% | 177.542105 | 71 | 56 | 66 | 46 | 40 | 45 | -25 / -16 / -21 |
| 156% | 180.227895 | 72 | 57 | 67 | 46 | 40 | 45 | -26 / -17 / -22 |
| 159% | 182.913684 | 73 | 59 | 69 | 46 | 40 | 45 | -27 / -19 / -24 |
| 162% | 185.599474 | 74 | 61 | 70 | 47 | 40 | 45 | -27 / -21 / -25 |
| 165% | 188.285263 | 75 | 62 | 72 | 47 | 40 | 45 | -28 / -22 / -27 |
| 167% | 190.075789 | 76 | 63 | 73 | 47 | 40 | 45 | -29 / -23 / -28 |
| 170% | 192.761579 | 77 | 65 | 75 | 48 | 40 | 46 | -29 / -25 / -29 |
| 173% | 195.447368 | 78 | 66 | 76 | 48 | 40 | 47 | -30 / -26 / -29 |
| 176% | 198.133158 | 79 | 68 | 78 | 48 | 40 | 48 | -31 / -28 / -30 |
| 200% | 219.619474 | 87 | 81 | 87 | 53 | 48 | 53 | -34 / -33 / -34 |
Conclusion
Hitstun decreases more compared to Smash 4 the higher the knockback inflicted is. This seems to be done not to intentionally worsen combos, but as a result of the balloon knockback (faster initial launch speed and decay as knockback increases); characters would be forced to fall for too long at high knockback values if hitstun was unchanged. Particularly harsh reductions also only happen at knockback where it's nearly impossible to combo characters either way, since they are sent too far; in other cases, the reduction is by only a few frames with hitstun canceling taken into account, which are easily compensated by characters' faster movement speeds (horizontal mobility, jumpsquat, and jump rising speed), lower landing lag, and decreased ending lag on some crucial combo moves.
Overall, while hitstun is lower, combos in Ultimate are not worse than in Smash 4; if anything, the severe nerfs to air dodges and increased overall damage (due to the 1v1 multiplier) grant more opportunities to set them up, and higher reward for doing so.