I'm have server with my friends on 1.15.2 and I have low-end pc, so my fps is around 40 and 50. I wish I could have more, It's really hard to play with that FPS. Thx!

There is an 1.15 branch you can build yourself or alternatively you can use mrmangohands/sodium-fabric which is an update version for 1.15. This isn't officially endorsed.

Sodium only supports the latest version of Minecraft. You can either use an unofficial fork as @FlashyReese suggested, or connect using a 1.16 client with multiconnect

Marking as a duplicate of #111 . As others have said, there are no plans to backport Sodium at this time. Anyone can fork and backport an unofficial version of Sodium under the terms of the license, so I would suggest you look into that.

There is an 1.15 branch you can build yourself or alternatively you can use mrmangohands/sodium-fabric which is an update version for 1.15. This isn't officially endorsed.

Thank you!