As @AmeliesUncle posted even without a cliff the subsidies go away when income gets high enough.
What's happening is something like this (it's complicated, the percentages change slightly each year, sothis is overly simplified):
Figure your modified adjusted gross income (MAGI) - https://www.healthcare.gov/glossary/modified-adjusted-gross-income-magi/
If over 400% of federal poverty level, then take 8.5% of your MAGI (The % is lower if you have lower income). That is your expected contribution to ACA premiums.
So if $100k MAGI, then $8.5k is your expected share of premiums. If $500k of MAGI then $42.5k is what you're expected to pay.
Now the ACA plan you choose and its cost doesn't matter. (Very hard to understand.) The metric is the second lowest cost silver plan (SLCSP) in your area for your age. See healthcare.gov for tools for find this.
Let's say the cost is $10k for your SLCSP. For $100k AGI your subsidy is $10k SLCSP - $8.5k (your "share") = $1.5k. For $500k MAGAI your ACA is zero because $10k premium - 42.5k (your share) is < 0.
The cliff used to be that if your income was > 400 % FPL then you got no subsidy. $100k, for example, would certainly not have gotten the $1.5k subsidy in the above simplified example.
Again this is a bit simplified.
There are calculators you can use to verify your numbers. For example,
https://www.healthinsurance.org/obamacare/subsidy-calculator/
https://www.kff.org/interactive/subsidy-calculator/
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