It looks like for now there is no version catalog support?

Not yet, but it is surely planned

Will this also solve https://github.com/gradle/gradle/issues/15383 for good?

It's unrelated. DCL is not for precompiled script plugins.

Oh, I see. I thought I could declare anything build related, including my own plugins for my own build, via DCL.

We have discussed allowing DCL in say

buildSrc

or plugins source sets but no commitment here and it might either happen in a long while or not at all.