It does mean that players on really old devices that don't support Vulkan will be left out, but Vulkan has been supported going back to some pretty old GPUs. You've got time though, as they'll be rolling out Vulkan alongside OpenGL in snapshots (development releases) "sometime over the summer." You'll be able to toggle between them during the testing period until Mojang believe it's ready. OpenGL will be entirely removed eventually once they're happy with performance and stability.
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