I've never written a use case before, honestly. However, here goes my first use case based on a Techopedia page:

I welcome your suggestions to the use case table please. Thanks.

Titles are plain text, so the plain text input is always a faithful preview of the result.

This request doesn't make sense, and I think assumes that titles support some kind of formatting, hyperlinks, etc. They do not. We link task titles in many contexts, and this would be confusing and/or inconsistent if they could also contain embedded hyperlinks.

@epriestley I meant that I can't preview new or changed titles while creating or editing tasks.

Titles are plain text.

The plain text in the input is always a completely faithful preview of the resulting title.

Put another way, there is no text you can type into this box which will render any differently from the text itself that is already visible:

To be completely clear:

• There is no way to make bold text.
• There is no way to make italic text.
• There is no way to make hyperlinks.
• There is no way to apply any other kind of formatting, styles, etc.

If there was a "preview" option, it would always show exactly the same text in the box.

Rephrased: if you take a photo of the screen using your phone, that is always a 100% faithful exact preview of the text. Since "look at it with your eyes" is a simpler version of "take a photo", we don't offer an explicit preview.

If this doesn't explain why we don't have a preview feature, can you show me an interface in some other piece of software which works like you want it to? In particular, show me some other piece of software which offers previews of a field that has no style or formatting behavior, where the preview is always exactly the same as the input text, and where no input I can type ever produces a different output in the preview (i.e., the preview is always an "exact photograph" of the input text).

I assume that some Wikimedia users who are new to Phabricator might expect a Phab task summary to allow some kind of formatting (for example, MediaWiki's StructuredDiscussions extension allows having links in the thread summary field), and that @gh87's intention is to allow users to realize that this is not the case.
However, I've seen enough Phab users ignoring the task description preview anyway when creating tasks (e.g. incorrectly expecting MediaWiki link syntax to work in Phabricator task descriptions)...

If the root problem here is something like "Users who are familiar with syntax X in tool Y may try to use that syntax when filing a task and be surprised that it does not work in Phabricator" and this is causing a substantial amount of confusion, a cleaner remedy might be to forbid the syntax outright:

diff --git a/src/applications/maniphest/xaction/ManiphestTaskTitleTransaction.php b/src/applications/maniphest/xaction/ManiphestTaskTitleTransaction.php
index 7dd9217760..0020205991 100644
--- a/src/applications/maniphest/xaction/ManiphestTaskTitleTransaction.php
+++ b/src/applications/maniphest/xaction/ManiphestTaskTitleTransaction.php
@@ -78,6 +78,15 @@ final class ManiphestTaskTitleTransaction
           $xaction);
         continue;
       }
+
+      if (preg_match('/\[\[.*\]\]/', $new)) {
+        $errors[] = $this->newInvalidError(
+          pht(
+            'Sorry! MediaWiki links do not work in task titles. Adjust your '.
+            'task title so it does not contain "[[ ... ]]".'),
+          $xaction);
+        continue;
+      }
     }
 
     if (!$errors) {

Of course, this prevents legitimate, knowing uses of whatever syntax you forbid (perhaps users reporting bugs with parsing the syntax elsewhere). A tailored variation of this might be to, say, raise this error for user accounts created in the past 30 days, and trust that users know what they're doing after that.

(There isn't currently a particularly easy way to prompt users with a dialog asking them to confirm a set of edits.)

I think this kind of change is unlikely to come upstream because this isn't a problem we see very broadly -- many tools don't allow formatting in titles, and I don't recall any other reports of user confusion around expectations that titles will support formatting or style rules. Because titles don't support any formatting, I worry adding a "preview" would be wildly misleading/confusing to users who aren't coming into the tool with a specific set of expectations about how formatting works.