Template obtains from jam01!: A template for Software Requirements Specification based on IEEE 830, and ISO/IEC/IEEE 29148:{2011,2017}. The non-functional requirement organization and language is based on van Lamsweerde's Requirements Engineering's taxonomy of non-functional requirements. The markdown format is based on rick4470's template which is based on IEEE 830.
There's also a Software Design Description Template forked from jam01 repository and translated by me.