It's stored in the dictionary, but ignored by PB dictionary reading code on the device.
Code:
$ readelf -CsW cramfs/lib/libdictionary.so | grep comparatorForQueryWord
334: ... pocketbook::morphology::OlenderStemSearcher::comparatorForQueryWord(...) const
392: ... pocketbook::dictionary::ChainingSearcher::comparatorForQueryWord(...) const
487: ... pocketbook::dictionary::CombinedSearcher::comparatorForQueryWord(...) const
611: ... pocketbook::dictionary::ExactSearcher::comparatorForQueryWord(...) const
614: ... pocketbook::morphology::HunspellSearcher::comparatorForQueryWord(...) const
920: ... pocketbook::morphology::LemmaGenSearcher::comparatorForQueryWord(...) const
The chaining searcher tries Hunspell, OlenderStem, LemmaGen and exact searchers in that order.
The combined searcher seems to be dead code; it is supposed to dispatch to other searchers based on locale, but there are no calls to constructor nor creation of that mapping.
There's no trace of morphems.txt being utilised anywhere, in modern firmware at least.