I’m trying to make a modulate dbuffer decal (which worked in UE4) but the modulate blend mode is now incorrect. White should be transparent… right?
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No, white is solid 
But if you’re modulating white on to the environment as a decal, white would be transparent. or else its not “Modulating”…?
This also worked in UE4, and “modulate” is commonly understood to multiply the colour onto the base
It’s not related to the albedo, it’s just how the opacity mask works.
Well it used to be related to the albedo in UE4. So whats with the change?
What would modulate affect in this scenario?
Absolutely nothing. Modulate when used for deferred decals behaves exactly the same as translucent blend mode.
One might logically expect that modulate blend mode for decals should behave as it does when used in the surface domain. That would make sense to me. But apparently Epic disagrees, because when it was reported as a bug it was flagged as “by design”
Given this, you might be tempted to ask the obvious question: If it behaves the same as translucent, and that’s how it is supposed to behave, then why even have modulate as an option? Isn’t that just redundant and confusing? 
Who knows
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That’s a really weird design choice…
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Indeed, this just isn’t right - if the Modulate mode doesn’t behave like it should anymore (multiplying the color), at least it should be removed to avoid confusion.