Mainly concepts but:

DFA and NDFA: learning about automata as a sort of format for problem solving is something I utilize frequently, when working on a complex problem at work I simplify it into an automata then start filling in the flow. This could just be a personal problem solving method, but I like to use it specifically because of that class and the way it was taught to me.

I draw these out almost daily.

Languages: building a regex VISUALLY changes the way you think. Regex actually does make sense if you can look at it as a language that consumes the characters. I probably write 3-5 regex's a week with more than 3 capture groups. Whether that is just for code refactoring or for actual string and data manipulation this has helped me get really quick at this.

Decidability: It is easier to verify a solution than to solve a problem. I guess you could argue this is just testing, however it's still useful to remember whenever working on any problem.

NP problem equivalencies: I'll probably get this bit of info wrong but the concept is solid, most problems can be re-worked as other problems (I think that was accurate). I.e., when you solve one problem you can manipulate your solution for another problem. I work A LOT with multi-relational db data, so I am constantly needing to find ways to traverse directed graphs that are connected in non-standard ways (different definition of an edge based on the depth from the initial node). I also am frequently re-writing code to identify cycles based on these dynamic (business problem) definitions. I always start with writing in peusdocode of a known problem and then work each step out for the specific problem.

Time Complexity: you say it's not fair to include, but a lot of good programmers are still very dumb and don't understand how to solve time complexity problems in the "real world". I hate reviewing code like this, if I see an N+1 problem I immediately send any code right back.

I guess that boils down to me using the problem solving skills I learned through the class more so than the concepts learned in the class. But I'd also argue I get stronger by weightlifting even though I don't directly do a bicep curl when moving groceries.