There’s no one-liner for this anymore. I opened an issue to track the feature request: Feature request: solve relaxation · Issue #1611 · jump-dev/JuMP.jl · GitHub. However, I don’t consider it blocking for the 0.19 release.
You can achieve the same result in a generic way by querying all variables from the model (https://github.com/JuliaOpt/JuMP.jl/issues/1612) and manually setting and unsetting integrality constraints (i.e., is_integer, set_integer, unset_integer, is_binary, set_binary, unset_binary around https://github.com/JuliaOpt/JuMP.jl/blob/10878c9fd513036510c0402e9f01402ca5dc12b1/src/variables.jl#L461).