The file over at https://github.com/vyos/vyos-nightly-build/blob/current/version.json no longer updates since the naming change of nightly releases.

Latest update is currently from 3rd march 2025:

[
  {
    "url": "https://github.com/vyos/vyos-nightly-build/releases/download/1.5-rolling-202503030030/vyos-1.5-rolling-202503030030-generic-amd64.iso",
    "version": "1.5-rolling-202503030030",
    "timestamp": "2025-03-03T00:30:48Z"
  }
]

Latest release with old naming:

1.5-rolling-202503040017

First release with new naming:

2025.03.05-0017-rolling

Latest release as of writing:

2025.03.09-0613-rolling