Hi Damien.
It seems that the link on the page is https://subsurface-divelog.org/downloads/Subsurface-v6.0.5214-CICD-release.AppImage while the file is named
Subsurface-6.0.5214-CICD-release.AppImage (no v anymore before the version)
This should be fixed now. Thanks for letting us know!
Ngā mihi
Michael Keller