Looks like this was a setting in the Sprout SEO Plugin for Craft 2, as a quick Google search for 'craft appendTitleValue' will tell you. Apparently, it has been removed in the version of the plugin for Craft 3.
I can't speak to the developer's motivation, but a likely explanation is that this is something you can and should handle in your templates. If you're not using the Sprout SEO plugin right now, you can just include the site name (or whatever you want) in your template:
<title>{{ entry.title }} | {{ currentSite.name }}</title>
If you are using the Sprout SEO plugin, checkout the documentation on template overrides for specific meta fields. The following code should accomplish roughly the same thing:
{% do craft.sproutSeo.meta({
title: "#{entry.title} | #{currentSite.name}",
}) %}