Yup, it’s Freeware since a few years now and has even spawned a brilliant standalone mod using its engine. Here is the link:
https://www.moddb.com/mods/tiberian-sun-client/
And if you want to check out the mod I mentioned, it’s called twisted insurrection. It delivers on many things that were lacking in the original, and has an extensive OST with some pieces by frank klepacki himself (made a lot of the music used in the original series). You can check out some trailers on YouTube. Mod Link: