Are the latest versions of ModelSim backward compatible with SE 5.8C ?
I'd like to see if designs made with later versions can be imported to this version.
I would say, only testbenches and macros related to tests, e. g. do files are "made with" respectively for ModelSim, the design under test is basically made for synthesis. Some minor changes can be necessary when preparing a design for simulation, typically ModelSim is more strict regarding HDL syntax or the requirement to initialize signals. But these aspects are independend of ModelSim version.
yes, It's true. Modelsim is more restricted in HDL codes rather than other simulators. But diffrent version of Modelsin are same in restriction. And if you have a code in old version modelsim it's so easy to compile it via others. On the other hand most of macroes are same.