Is there a go-to vendor/library that handles this (OIDC, SAML, SCIM) for SaaS services these days? Just like how everyone uses stripe for billing, everyone uses <vendor> for auth?

(self plug since you asked!)

WorkOS does exactly this. It's "Stripe for enterprise features."

https://workos.com

Our customers include OpenAI, Anthropic, xAI, Cursor, Perplexity, Vercel, Replit, Webflow, Clay, Hex, Carta, Plaid, Drata, Vanta, and many others. If you've used these products, you've used WorkOS!

WorkOS makes it easy to "cross the enterprise chasm." Here's a bit more of the backstory: https://x.com/grinich/status/1841569664465568248

We also launched on HN 5 years ago :) https://news.ycombinator.com/item?id=22607402