This isn't going to provide you with any help, but the only thing I can confirm is that I still see the import legacy option in all of my environments. I don't recall seeing anything that this was being deprecated. I searched Release Plans Visualized plan and don't see anything for deprecating it. To use the feature, Dataverse needs to be enabled in the environment. Have you confirmed the environment where you want to import the solution has Dataverse enabled? Since the import solution option is there, it would appear it is.
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