Look at the 2nd-last Changed item in the release notes:

https://www.mozilla.org/en-US/thunderbird/52.0/releasenotes/

Thanks for the quick answer. How is the documentation for this going to be managed? Is it expected that everyone should be using the latest version of the software and the documentation isn't up to date yet?

It is going to take this old dog a long time to learn this new trick. I've been using Ctrl + L for a long long time.

I notice on the keyboard shortcuts page there is an 'Edit Article' option under Editing Tools at the upper left, so you can submit changes for review if you wish.

Ideally, changes in TB would be relayed to the people responsible for documentation in advance of a release, especially since major releases arrive about once a year.

Great, good to know. Now how do I change it back?

there is an add-on @ https://addons.mozilla.org/en-us/thunderbird/addon/dorando-keyconfig/ for custom keyboard shortcuts, but I'm having difficulties configuring it... I think this is a no-go to adapt TB shortcuts to the ones of Office. Office users mostly use Outlook anyway, not TB, so... My suggestion for the next release: contextual sensitive shortcuts: • Ctrl-K leave it as «insert link» • Ctrl-L restore shortcut as «insert link» if text is selected in the editor (now, nothing happens). If an e-mail is selected in the list, then Ctrl-L forwards the e-mail as it does now (Ctrl-F can't be used because it stands for «find»).