Drupal - "admin/content" filters not available anymore

You need just to install the Views module that is part of Drupal core. Once you install it, that page appears as the following, which is how I see it in my site.


If I uninstall the Views module, that page appears like the following.


Now that the Views module is in core, there are many pages that have been converted into views. As you see from the first screenshot, there is a new Files tab which is a view.

If you don't see the Views module listed between the modules, download again Drupal, because that means the archive used for the installation is missing some Drupal files, or they have not been correctly copied.

You might have deleted the Content view by mistake. To re-import it, setup a new or another Drupal 8 site. In the new site goto Configuration > Configuration synchronization > Export Tab > Single Item, Configuration type: View, Configuration name: Content (content). And then copy the contents of the "Here is your configuration". In your original site goto Configuration > Configuration synchronization > Import tab > Single item, Configuration type: View and paste into "Paste your configuration here" and press Import.