@CG Thanks for reaching out!
If you think your boots may have a little life left in them for skiing (even without the liners), the best thing you can do is donate them to a local second-hand sporting goods store or the Goodwill. Unfortunately, if the boots have no "ski life" left in them, you'll have to throw them away. You may want to reach out to your local recycling department and see if they have a program or options for recycling hard plastics, as that is what makes recycling ski boots challenging.
Thanks for the question and for thinking critically about how to dispose of your boots!