The ignorePlurals parameter can only be set in the Algolia Dashboard and cannot be altered through the Shopify plugin. This article has a full list of which settings are handled by the Shopify integration and which by the Algolia dashboard.
ignorePlurals is used in conjunction with the queryLanguages setting. You should always set queryLanguages and indexLanguages for your indices. If you don’t, the engine uses either the default (all supported languages) or the list of languages specified in ignorePlurals and removeStopWords. This can lead to unexpected search results.
Our documentation on how to Set an index's query language will guide you through the steps to set these parameters in the Algolia Dashboard. For more information on ignorePlurals and queryLanguages we recommend our guide on Language-specific configurations.