So let's do the calculation based on one of my old logs of a +21 Tyrannical where we did 892M on October 12 (Patch 7.3).
94.1 million * 6.73 (+21 modifier of +573%) = 633.3 million * 1.4 (Tyrannical modifier of +40%) = 886.62 which seems about right. So at least the first boss of Vault's health hasn't changed.
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And looking at a Base Mythic Nelth. Rokmora has a base 111 million health. Looking at the earlier linked video is 1 billion-ish.
1,000,000,000/6.73/1.4 = 106 million which seems about right.