Add UI text indicating that the current data may be stale or incomplete

This message can also mention this is a prototype & feedback is appreciated. While keeping it brief & readable.

Maybe we can use the Plugin's labels and descriptions for this?
$this->enabledSource->getCurrentSource()->getPluginDefinition();

We could use the above in "BrowserController.php" and that will return the label and the description of the plugin (which comes from the annotation in the plugin class) and pass it to the front-end.

So, for example, when using the mock, it will look like this: Drupal.org (mocked): Gets project and release information from a mock API

I like that idea. I think that it makes sense to allow each plugin to provide its own helper text?

I would pitch the text as:

Title: "Drupal.org Mock API"
Description: "Provides a mock implementation of data from Drupal.org. Though the data looks real, it is a sample of data pre-populated from a fixed data set. It may show stale or incomplete data."

Then I think show it... maybe above the keyword search as like:

Data Source: Drupal.org Mock Api
Full description here

While the merge request does exactly as requested in the comment in #5, it doesn't speak to the site builder at all as it is very technical.

How about something more like this? Included is the word prototype and call for feedback as recommended in #2.

Project browser is currently a prototype, and the projects listed may not be up to date with Drupal.org. For the most updated list of projects, please visit https://www.drupal.org/project/project_module

Your feedback and input are welcome at https://www.drupal.org/project/issues/project_browser

I prefer the language proposed by @thejimbirch in #11. I'm also aware that message phrasing can quickly get bikesheddy.. and I'm also aware that this is one of the more set-in-stone MVP requirements. SO. I propose the following:
If there's reasonably fast evidence that other people agree the language should shift to #11, lets quickly make that change and get this in.

If there isn't clear support, or it's being debated (which is reasonable), lets move that discussion to a different issue and commit this one as-is so the MVP requirement is addressed. Basically, I like the suggestion but I want to avoid delaying the MVP on it so if there's not quick agreement, a followup issue is where it can get the attention it deserves.

I like #11 as well.

I thought of some suggestions to improve it - but I think the wording in #11 is just fine. Lets run with that is my vote.

I prefer the language in #11. Even the warning looks scary. I would prefer it to be more plain text just in italics maybe? But again, don't want to bikeshed. Something is better than nothing, and I'm OK if we ship it as-is with #11 text.

I agree that #11 is better than #9 for our target audience. We can ship it like this and update later if needed. The intent of letting folks know that the data may not match that on drupal.org is the key

Updated to use suggestion in #11 and changed from warning message to status message

Based on the comments above, and feedback in Slack, I am moving this to RTBC.

Great job @hooroomoo!