It is now a known fact, that Joomla will drop legacy support (well, sort of) in future versions. As you may figure, this was not an easy call. Dropping Legacy support is not just a matter of Joomla! Development Team stop supporting legacy frameworks, it is a process that needs to be understood by third-party developers and even more than that - be applied to third-party extensions.
Joomla! cannot move forward while leaving 3PD's floating on air...

I will not present to you any statistics on this matter, but we are on J!1.5.4's eve, three months after J!1.5.0 was released and a lot longer since J!Framework was announced (and even completed) - as you can see in the JED, not all 3PD's rushed into migrating their extensions to J!1.5.x. Some, fear of the process, some are not familiar with it, and some just don't care or have time for this - but who am I to blame? I can relate to them, even major extensions have not migrate yet, so why should they...? why get into fork development, or duplicate code to support both J!1.0.x and J!1.5.x?

Moving the power to the community

So I decided, that if developers won't do the job, community will & should. I'm not expecting common users to start developing and/or migrating extensions, I'm just expecting them to be aware of what they are using - specially if it is an extension requiring the use of Legacy-Mode.
Why is that important? because now, community users (or at least site administrators) will be able to just urge those 3PD's to migrate their extension into a Native J!1.5.x extension. This will help Joomla! in dropping Legacy-Mode, which in return will help you back by allowing us to focus on moving Joomla! forward...

Legacy-Marker

When a new extension is installed, com_install already notifies the administrator if he needs Legacy-Mode to be ON in order to install the extension. What is not there, is the ability of Joomla! to actually tell the administrator at any other time, which extensions require Legacy-Mode and which don't.
Let's assume the site administrator decides to turn OFF the Legacy-Mode (I won't even try to answer the question: Why?).

We obviously have 2 possible scenarios:

1. The good - All installed extensions are J!1.5.x Native, the system runs in Native mode.
   Technically speaking, Legacy-Mode is not required for this site to function.
2. The WCS - There is at least one extension not written for Native J!1.5.x, the system will simply break
   Technically speaking, Legacy-Mode is required for system to function.

The twist

The solution I introduced is quite simple. A small change to the com_installer's model, to record the Legacy state of an installed extension and an addition of one new column in the component's view to display the Legacy state in the corresponding list of extensions (Templates, Components, Modules, Plugins - Languages did not get this change, because Joomla!1.5.x does not accept J!1.0.x languages to be installed anyway...)
Two new strings were added to en-GB.com_installer.ini in order to allow mouse hovering on the current Legacy state of the extension. This will practically tell the viewer, to contact the extension's developer for further support on a Joomla!1.5 Native version of the extension.

An example

Here is a little example of the docMAN component (version 1.4RC2), installed on Joomla!1.5, where the mark shows that docMAN still needs Legacy-Mode to be ON.

(this is only for the demonstration purpose of the Legacy-Marker, no bad intentions meant towards docMAN team)

When to expect it?

This Legacy-Marker was committed into the core, before the release of J!1.5.3, but due to a bug in the model, it will be usable only after J!1.5.4 is released (or getting the fix from the SVN)