Question: Was firmware update 3.21 optional or mandatory if users wanted to continue accessing the PS Network?
Sentence: The 'OtherOS' functionality was not present in the updated PS Slim models, and the feature was subsequently removed from previous versions of the PS3 as part of the machine's firmware update version 3.21 which was released on April 1, 2010; Sony cited security concerns as the rationale.
Answer in sentence: no