ACSL v18 release is not present in the ACSL github releases page
The frama-c download page for ACSL: https://frama-c.com/html/acsl.html lists a V.18, but this version is not present on the ACSL github site: https://github.com/acsl-language/acsl/releases where v17 is marked as "latest"
I do see that version.tex says v18 - so perhaps no one pushed the button to publish a build of v18.