The safest way to archive data from any Maximo tables, including the workflow tables, is to use the Optim archiving tool. This link will give you more information on it:
https://www.ibm.com/support/knowledgecenter/en/SS9NTS_7.6.0/com.ibm.arc.doc/pdf_arc_qsg.html
Directly deleting records from tables is not recommended.
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