is DebugClassLoader the only debug utility we need to provide ?

I think so. At least we don't have any other.

I have just one concern about discoverability. Is this something we should take care of?

Aren't we regret this in the future if we create new debug utilities? 😕

Also, having the DebugClassLoader in a component that was created to render PHP errors / exceptions in different formats does not seem consistent.

That's still the most pragmatic way to solve the issues that the linked issues describe.
And having a component with debug utilities is just normal (while having a Debug component with non-debug things is not.)

But that still doesn't solve the problem for existing projects, right? 🤔 Their index.php is still referencing the Debug component but it's still not required anymore.

As with any regular deprecations. The issue is different, see linked issues. (Sorry I can't elaborate I'm on mobile and about to be AFK)

I thought the whole point of creating the ErrorCatcher component was to split the debugging part and the part that is required even without debugging turned on.
Otherwise we could have just added the new ErrorRendering inside the debug component (or any other component like HttpFoundation) and wouldn't have to deprecate anything.
So by moving everything into ErrorCatcher, we achieve nothing but have a huge number of deprecations and BC breaks without any gain. Big 👎 for me.

The main point was to improve and split the rendering part actually. Splitting the debug tools wouldn't be of much interest to me.
So, in the end, in think we need an ErrorHandler component, that contains the debug tools too, but NO rendering at all.
Then rendering would be split in a dedicated component, and we would deprecate Debug and drop ErrorCatcher.

But that still doesn't solve the problem for existing projects, right? thinking Their index.php is still referencing the Debug component but it's still not required anymore.

@fancyweb we could use the flex command symfony:sync-recipes to update the index.php file with the new namespace, I know, that will require an extra effort and should be informed as well.