Deubiquitinase that plays a key role in the spindle checkpoint by preventing premature anaphase onset. Acts by specifically mediating deubiquitination of cdc20, a negative regulator of the anaphase promoting complex/cyclosome (APC/C) (By similarity). Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). Belongs to the peptidase C19 family. USP44 subfamily.