When it comes to SSO, "buy" instead of "build", and focus on the core value proposition of the service with that saved time and resources.
We haven’t built an API for it though, instead opting for a UI that walks the end-user through the steps of integrating with their IDP. That’s partially because every IDP is so different that we felt you really need a UI to show exactly what to do.
It's essentially a hosted UI that allows end-customers to fully configure SSO/SCIM/etc.
Demo: https://demo.workos.com/
(I work at WorkOS. :))
To adopt an OSS approach for SSO, the startup still needs to dedicate resources to researching the OSS options, deploying the OSS, integrating it, and most importantly, operating it.
Once the startup gets into the weeds of integrating SSO functionality with customer's IdPs, in the limit, the cost of the effort will start to approach "Build".