desibilator is a free Audcaity-specific De-esser plugin,
it’s capable of surgical-precision, but does not operate in real-time.
ToneBoosters have two free De-esser plugins which work in real-time:
One is simple (2 band) “TB_DeEsser_v3” , the other called “sibalance” [sic] is complex.
Another free option is “t-de-esser-plus” …
it’s a cut-down version of their “Pro” paid for plugin.