DX12 is key for all UE5 features to work correctly and I think it will struggle on anything but next gen GPUs. Without it, Im not sure it will work by design at all.
But this is why we have packaged the project with the video settings menu, so one can push even a potato if they wanted too. Im interested to see how it fairs on older tech. How old is no good? I think if the GPU is not DX12 compatible, then its really a ''best of luck'' situation.
My R9 280x is compatible with DX12 https://linustechtips.com/topic/346646-will-the-r9-280-support-dx12/
Also i can play recent games like Dying Light 2 in DX12 mode as well (but with performance issues). For example i can run fine Unreal Racing here https://kronbits.itch.io/unreal-racing