I would like a 1.21.5 version as for some reason vulkan mod is still on 1.21.5 and updates come out super slowly

The performance in 1.21.5 is quite bad compare to the previous 1.21.4, this is slightly improved in 1.21.7 and 1.21.8, consider upgrading :)

Use old versions if you want to use 1.21.5. No new update is planned for 1.21.5.