Yes it is, as it is about the semantics of type hierarchies, not their syntax. If your software has type hierarchies, then it is a good idea for them conform to the principle, regardless of whether the implementation language syntax includes inheritance.

It might be argued that CLU is no better than typical OO languages in supporting the principle, but the principle is still valid - and it was particularly relevant at the time Liskov proposed it, as inheritance was frequently being abused as just a shortcut to do composition (fortunately, things are better now, right?)