Rerouted (renamed, verified). The GitHub user or repository was renamed. The tag is still there, at the new name. The Lamdera package server serves a copy recovered from https://github.com/CancerAidDev/elm-form-builder.
What we checked: The repo moved to github.com/CancerAidDev/elm-form-builder. Rebuilt with git archive from commit dcc63cf407f3 of https://github.com/CancerAidDev/elm-form-builder (zip prefix CancerAidDev-elm-form-builder-dcc63cf/, America/Los_Angeles times): the zip is byte-identical to the one package.elm-lang.org recorded, SHA-1 96f55a9dc679f4fd1f1de2124ef9a38334252d6c.
The Lamdera compiler uses this copy and prints a one-line notice saying so. elm itself fails to download this version.
Details · package.elm-lang.org's zip: https://github.com/canceraiddev/elm-form-builder/zipball/3.0.0/
Uses Iso8601.fromTime produces a date with timestamp and takes first 10 characters to return "YYYY-MM-DD"