What molecules can be eliminated in condensation polymerization?
As an abstract answer, to me it would be whatever is commercially viable to manufacture and sell. Commercial polymerisation processes eliminate ethylene glycol (in the manufacture of PET) which is probably recycled,  methanol in transesterification reactions, ethene or propene in metathesis type,  phenol (recycled) in the use of diphenylcarbonate with bisphenol A, etc. In home applications,  acetoxysilicone sealant gives off acetic acid. So, anecdotally, it seems "simple" molecules of MW around 100 or less might meet your criteria.