I am having a hard time following you because you are asking about things that are not part of Showcase... so please bare with me....
When adding a new Showcase Item, part of the process is selecting the Showcase Category that the Showcase Item will be associated with.
Each Showcase Category has a configuration setting called "Automatically create thread in forum". If that configuration setting is SET (set to a Forum), then when creating a new Showcase item in that category, during the save process, the Create Item Service will automatically create a new discussion thread in the forum that you set in that configuration setting. The TITLE of the newly created thread will be the SAME as the Title of the Showcase Item (it uses the newly created showcase item data for setting things like Thread Title).
There is NO OPTION in Showcase to set a "custom title" for the automatically created discussion thread. The process uses the Showcase Item TITLE to set the Discussion TITLE.
The thread-title will be generated automatically ... but where???
There is no such setting in Showcase. I am not sure where you are getting that from? Showcase has never had such a setting.
If you create a new Showcase Item with the Title "This is a showcase item title" and the category that you are adding the Showcase Item to is set to automatically create a thread in Forum "X", then create service for the showcase item, will run some post save functions that will create a new discussion thread in Forum "X" with the Discussion Thread TITLE "This is a showcase item title".
That same process also creates the 1st post of that thread, which is some simple text with a link to the Showcase Item (which also uses the title of the showcase item at the time of creation). You can change that text via the phrase xa_sc_item_thread_create
Note: once the post is created, its static text just like all other posts (meaning that if you change the title of the Showcase item, its not going to change the title of the showcase item in the first post as the post is RAW TEXT, its not dynamically fetching the content (you'd have to actually edit the first post to change any of the text, just like with any other post).