In early 2011 I started to use DT, and soon added keystroke opt-L to Format>Add Link…
Later I didn’t use DT that much, until now, and found the keystroke no longer worked: I got the line continuation character instead. I tried changing ellipsis to 3 dots (it’s 3 dots, in 2.4.2), but that didn’t help. Eventually I got it to work with cmd-opt-L.
I suspect it’s the result of something Apple changed, but either way, I’d like to understand what it was, in not too technical terms.
Nothing changed. When pressed, the option key on Mac keyboards activates alternative characters for most keys. (For example, the “line continuation character” (¬) is option-l, as you pointed out.) This image shows the keys that are active while option is pressed on a MacBook Air with the U.S. input source chosen. The orange keys are diacritics. Apple keyboards have operated this way since at least Apple II. (Probably earlier?)
For that reason, keyboard shortcuts that combine “option” and a standard key, without an additional modifier (i.e., “shift-option”) usually do not work as expected.
If you were once able to create a shortcut using “option-l” it was probably serendipitous that it worked. DEVONthink has always worked with the standard Apple keyboard behavior, as far as I can recollect.