Supporting older WordPress versions in a Plugin/Theme? [closed]

The practice of only supporting latest WP version seems popular in WP development circles, yet there is massive percentage of blogs running outdated versions. Not supporting those plugins seems it can lead to significantly less usage than if (some) older versions are supported.

Support of older version comes with major issues, especially:

• Complicated testing for multiple versions (unit tests?);
• No access to latest functionality or having to backport it;
• Incompatibilities of your code between WP versions (works in one, breaks another);
• Wider hosting requirements range (older PHP and MySQL versions, dropped by WP).

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1. In your experienced had you ever had to support older WP versions in your code?

2. Which organizational and technical practices had produced good results?

3. What type of code do you use to manage the multiple version support.

Supporting old versions of WordPress seems reasonable, but is it really necessary? Personally, I think supporting old versions is waste of efforts and time and it prevents end-users from updating their WordPress installs. What’s the point of being a WordPress developer if you’re encouraging users to remain using insecure and out-of-date versions?

Just adding a function that checks the version of the DB/WP version and displaying a warning message if the site is running outdated version will remind the user to update; everybody knows how to update WordPress these days–just a couple of clicks and WoW, WP is running the latest version.