Is SEJPME ( Senior Enlisted Joint Professional Military Education) Phase I still a thing?

It's still on JKO. You have to search under "curricula" instead of "courses."
When you login, you will see a link for SEJPME-US001 - Senior Enlisted Joint Military Education (SEJPME) I Curriculum (40hrs) on your My Training tab. Click it for a walkthrough on how to find and enroll in the course. If you still have issues, shoot me a message on Facebook and I'll walk you through it.

I believe that you can only take the Phase II, the Phase I (I believe) was discontinued when the course was revamped.

Its no longer required as a pre-requisite to Phase I or required to take during the SMC-DL.