I have not seen any further mention of the 2021 project outside of the initial proposal, but didn't forget about it.
My assumption is that the money for it (since Disney and LegCo) did not increase the budget, was that it wound up being added into Arendelle for the coaster.
A fantasyland change is a reasonable guess, this is basically the Shanghai apology tour expansion project. I had also thought it was Toy Story Midway Mania (not realizing the wording was that it was a reimagined attraction and not just another new attraction).
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My further assumption is that everything shifted 9 months, Frozen ate the 2021 attraction budget and Marvel still comes on time for 2023.