TYPO3 Core

I think this is more a must from your described perspective.

Patch set 1 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60504

As mentioned, everything beyond the TLD is managed by the actual server software (apache / nginx), Apache can be configured to have both variants allowed, so:
a) TYPO3 v9 generates URLs in lower-case by default
b) if your server is not configured to treat both URLs the same way, you're doomed.

In our case of our projects, we built a PSR-15 middleware to overcome this issue, and redirect to a lower-case variant.

Maybe you want to try out this extension? https://github.com/b13/justincase

The extension was just published to TER, and works just fine, solves the issue and is compatible with TYPO3 v9 and TYPO3 v10. I recommend using the extension, as this only affects
a) TYPO3 installations with URLs that are communicated in a wrong fashion from external people
b) RFC states we (TYPO3 Core) should stay case-sensitive, as per "SHOULD" definition - maybe I will change my mind but I think we're good for now.

In general, I assume we need to consider how to "create" slugs for pages that they could be upper-case if requested, but that's a bigger and separate topic.

For reference:
https://github.com/b13/justincase
https://extensions.typo3.org/extension/justincase/

If you feel we should keep this issue open for some reason, let me know.