Announced by publisher Live Wire, the reasoning provided for the delisting is that the licensing agreement for the Switch version has expired. It seems that rather than attempt to renew the agreement, the best course of action for Live Wire at this time is to remove it from sale entirely.
Of course, you do still have a month to purchase the game, at the time of writing. Should you choose to do so, it will remain in your library and you can download it whenever you want. Still, it's a bit of a blow for preservation considering how rare the Limited Run physical edition is these days.
Priced at £15.29 in the UK and $19.99 in the US, the game has been included in multiple sales in the past, so hopefully it'll be discounted again before its removal, but we're not betting on it. We'd probably recommend picking it up if you're a fan of shmups before it's gone forever. The clock's ticking.