how do i get legacy version (2019.11.12) of superliminal on steam? so I need it for legacy practice mod. i have only 1.0 beta version in steam

Once you downgrade to beta 1.0 in Steam, you can download the downpatch here and extract the files into the game folder. Then launch the game through Steam as usual.

Keep in mind that the legacy version is quite different from the current one, and a glitchless run on that version will end up in a different category.