REST views: Default format no longer works

Problem/Motivation

[Setting this as a major, as it may cause breaks on live sites. Also not 100% sure if this is a proper bug, or just something that should not have worked before.]

I have a view over a content type with a REST Export display on it. This particular view exports data as an XLS download via the XLS Serialization module. The display has a path defined on it, /export/mydata.xls Since I need my user's to do this without my help, I also have a menu item for /export/mydata.xls so they can download these reports whenever they want (I have a somewhat related entry about REST Export displays and menu entries somewhere). Other than the master, this is the only display on this view.

This has been working happily for a long time (potentially as early as 8.0.x as this was my first real site in this branch).

When I updated to 8.5.0, the display broke,

Symfony\Component\HttpKernel\Exception\NotAcceptableHttpException: No route found for the specified format html. in Drupal\Core\Routing\RequestFormatRouteFilter->filter() (line 55 of /var/www/mysite/web/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php).

I had to update the menu item to /export/mydata.xls?_format=xls (can't do this on the display). Not a big deal, but a little confused why this is happening now. This could be related to #2858482: Simplify REST routing: disallow requesting POST/PATCH in any format, make consistent, but I am not 100% sure.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes