It is highly recommended that you take regular backups of your metadata store, since it is difficult to recover Druid clusters properly without it.

This is a recommendation that accompanies the documentation on the old segment scan-and-insert tool. Does this still hold true? That is, is it realistic to reconstruct segment metadata by scanning deep storage?

Reconstructing the metadata from the segment files is no longer supported, as the required metadata is no longer stored with each segment file. So, it holds true that you should be taking backups of your metadata db as recovery without it is not possible.