So, I know that you can redeem Bedrock if you have Java account before/ on 2018. Will it be the same like if you have java account before/on 2020, you can redeem Dungeons for free?
Minecraft Dungeons is a completely different game, so no.
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Also it's too late to get the free Bedrock, even if you bought Java a long time ago.
The offer ended a while back.