I react with the changes on the plugin.
1. I think it's not at all professional to update a plugin without check the changelog. It's written to multiple times as features are deleted.
2. If features are removed, is that they are more efficient and responsive. When Simple Tags has been created in 2007/2008, there was no valid alternative to related posts.
3. I think that wanting to include too much functionality into a plugin is a bad thing. This complicates the development generates more bugs and slows the rythmn of releases.
I do think I have kept the template functions to reduce errors template, however there is the function function_exists() to call functions in the community plugins.
I will work to restore the page "Manage tags", but I'm not going to restore the related posts or related tags.
I'll just add empty functions to avoid error messages.
There are plugins that do very well what you do: Similar Posts, YARP
And there are also manual solutions with query_posts().
I recall that this plugin is written in my spare time, since the introduction of the extend/plugins, I received no donation on this plugin. Made donations, and I will devote more time to restore, develop and evolve the plugin.