How can I determine the maximum supported resolution on a given Macbook? I recall that Macbooks produced in 2009-2011 supported a maximum external resolution (through Thunderbolt or DVI) of 2560x1600.

An aspect ratio of 16:10 is much more useful for those who use their Macbooks mostly for work, rather than entertainment. The aspect ratio of 16:9 is more suitable for entertainment.

Yet Macbooks produced starting in 2012, and before supporting 4K external resolution, appear to provide only 2560x1440 (which is 16:9).

It's really a pity, because this may be one factor why it's impossible to find 2560x1600 monitors in the 24" and 27" variety. To get 16:10 one must move all the way to the cumbersome 30", and then the DPI is hardly impressive.

But before buying any monitor for a "legacy" Macbook, it's nice to know for sure.

How do I determine the maximum external resolution supported on a given Macbook? Is this available in System Information...? I see the type of graphics card there, but not the resolution. Under Thunderbolt I see the speed (10GB/s x2), but not the resolution.
While I don't normally like to post 'link only' answers, this needs to be an exception.  The answer to your question will largely be 'it depends on the MacBook'.  

To help you figure it out, I recommend going to EveryMac.com's MacBook Pro section.  This site will list the technical specs of every MacBook, including the maximum external resolution.

For example, here's a partial tech spec for a 2010 13" MBP: