If you've been using Improved Layered Navigation extension by Amasty for Magento 1 and now are going to migrate to M2, please pay a careful attention to your URL settings.
To adjust the URL scheme, please, go to the Admin Panel and then proceed to Stores -> Configuration -> Amasty Extensions -> Improved Layered Navigation: SEO. Expand the SEO URLs section.
Please, configure the following options to modify the URL building logic:
Include Attribute Name — select 'Yes' to add the attribute name to the the URL scheme, e.g. black → color_black.
Filter Key — the option allows to add a filter key to the URL scheme, e.g. black-xl-activity → shopby/black-xl-activity.
If you have URLs working based on 'Long with URL key' setting in Magento 1, then, please, configure the following options to migrate to Magento 2 and keep the existing URLs:
Include Attribute Name = Yes
Filter Key = shopby (example)