The Terratonic datapack (at https://modrinth.com/datapack/terratonic) is provided for compatibility with Terralith. However, the Modrinth page doesn't seem to have a version compatible with newer versions of Minecraft.

Using the two mods together still works like normal.

Is there an expected time for Terratonic for 1.21.4 to come out as a standalone datapack?

Terratonic already works on 1.21.4, I've just neglected to bump the supported versions on Modrinth. Doing that now.