Because parse_str replaces e.g. dots by underscores, Symfony replaced parse_str by custom processing, see.
https://github.com/symfony/symfony/pull/20101

The = separator is now > to make it even less ikelly to collide with a char in some path. The above example should be updated to:
subl://%f:%l&/foobar/>/home/toto/&/baz/>/home/titi/ remaps both /foobar/ & /baz/.

Where would Xdebug get this information from?

From the existing xdebug.file_link_format setting itself!

Sorry, I still don't understand what you are asking. What is that extra stuff supposed to do? The file link format is rendered by Xdebug for whenever a file/line number is shown in HTML.

Let's say xdebug.file_link_format=subl://%f:%l&/foobar/>/home/toto/&/baz/>/home/titi/

The first "&" after the last "%f|%l" starts a map, where keys and values are separated by ">", and key-value pairs by a "&". In the previous example, the map is:

• /foobar/ => /home/toto/
• /baz/ => /home/titi/

This map defines prefixes that should be replaced in the rendered file.
Eg if file is "/foobar/abc/def", the "%f" will be rendered as "/home/toto/abc/def".

This is useful on VM/container setups, where paths inside the VM have a different prefix from the IDE pov.

For reference, the logic in Symfony is implemented by this class:
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/Debug/FileLinkFormatter.php

IMO, it's up to IDEs to implement these mappings. And many already do so.

PhpStorm: https://www.jetbrains.com/help/phpstorm/2016.3/deployment-mappings-tab.html
Netbeans: http://forums.netbeans.org/ptopic53513.html
Eclipse PDT: http://www.eclipse.org/pdt/help/html/adding_a_server_path_map.htm
Sublime Text: https://github.com/martomo/SublimeTextXdebug#features

Xdebug never does anything with path mappings itself, as it doesn't know about them. I don't think it should do this depending on a setting that is supposed to format a link. Having this information both in PHP configuration, and in an IDE, is double up and likely going to be confusing — setting up path mappings is confusing enough for many people.