The sk1 file extension is for sK1. This is free Linux illustration software. It stores vector drawings made in sK1. sK1 is now obsolete. It was replaced by PrintDesign. The sk1 file format is no longer supported. It is not used now. It is likely obsolete. Old system files often become obsolete. So do old software file types. And old versions of files replaced by newer versions.
The default software for sk1 files is sK1. It shows sk1 vector drawing files. sK1 is open source Linux software. It is an alternative to Adobe Illustrator or CorelDRAW. The sk1 project was frozen as the toolkit was outdated. PrintDesign succeeded sK1.
sK1