From Springer:

“On July 25th, the SpringerProtocols.com platform will be deactivated. You may have noticed that SpringerProtocols content is already searchable via our new research platform Springer Nature Experiments, our advanced research solution dedicated to protocols and methods, while the actual SpringerProtocols content is hosted on SpringerLink. Free-to-use, Springer Nature Experiments has been specifically designed to help researchers find and evaluate protocols and methods across the whole Springer Nature protocols and methods portfolio”