If you sign up for the new developer workspace, it’s language is confusing and says you need to part of an “organization” as if it needs to be an officially recognized company. But you can ignore that and just be your own organization and get registered. You should be able to use that for writing new SmartApps and device handlers using the new API. You have to have your own hosting for those.
And your custom Groovy code (running in the SmartThings cloud) will need to be rewritten to use the new API next year when the Groovy infrastructure (and the current IDE) is shut down as part of Phase 3 (Announcement | Changes to our Legacy SmartThings Platform).