gopls v0.18.1 (latest at time of writing) does not recognise the value of GOEXPERIMENT when set to GOEXPERIMENT=synctest as instructed by the godoc. The server emits the following error and then fails to start, meaning you cannot use LSP features whilst trying out synctest.
LSP[gopls] Error loading workspace folders (expected 1, got 0)
failed to load view for file:///Users/<name omitted>/go/src/<path omitted>: err: exit status 1: stderr: go: unknown GOEXPERIMENT synctest