It seems there are a lot of symbols which are private and not a part of the public ABI that are exported. Those include _hashtable_*, blake2b_ (when the bundled b2 is used), and probably some of rs_* functions (at least some of rs_* functions were exported in 1.0.0 but are not exported anymore because they were made static but SONAME was not changed so I assume they were private from the beginning).
Setting all symbols as hidden and only explictitly marking public ones as visible helps maintaining ABI correctly and understanding when the SONAME must be bumped.