Hi Pauline,
To clarify my previous response, the free version has never offered the option to rewrite individual product category permalinks or fully customize their permalink formats. The free version included and still includes an option to overwrite global trailing slash settings, which also affects product category permalinks.
By choosing the “Use default settings” option, the plugin will follow the trailing slashes mode defined in the permalink settings and will not overwrite the built-in trailing slashes settings:
https://permalinkmanager.pro/docs/plugin-settings/how-to-add-remove-trailing-slashes/#native-settings
By default, WordPress uses the “Custom Structure” field in the Permalink Settings to decide whether URLs include trailing slashes. If the field ends with a slash, WordPress adds one to all URLs. If there is no slash at the end of the field, the URLs will not have a trailing slash.
Furthermore, if “Use default settings” option is selected, the “Trailing slash redirect” functionality will not be active (it works only if either “Add trailing slashes” or “Remove trailing slashes” is selected in the plugin settings).
Neither the WordPress built-in permalink system nor Permalink Manager allows for different trailing slash settings for different content types. The setting is applied globally, so it is not possible to remove trailing slashes from one content type, such as product categories, and keep them for other URLs.
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If there any backlinks for product categories that end with trailing slashes + the trailing slash redirect does not work, both the URL with the slash and the one without will be accessible. Could you confirm whether the redirect works for both versions of any product category URL?
https://example.com/collections/product-category-name
https://example.com/collections/product-category-name/
To verify if the problem is caused by Permalink Manager, try to deactivate the plugin temporarily and check if the issue persists. The plugin only adjusts permalink settings while it is active and does not overwrite them permanently.
Best regards,
Maciej