I'm in the position where I have inherited a highly complex codebase predominantly written by developers who have since separated from the company. I am not aware of what circumstances they separated under. The code in question is several years old, and ideally the company would like to update it to take advantage of new solutions that the old developers largely had to home-brew at the time.
The code is mostly uncommented, and certain parts are wholly undocumented. The developer in question maintains a pretty sizable social media presence and is still involved with software engineering at other companies.
My question is, is it appropriate to get in contact with this previous developer to get some insights into the code, and if so, what would be a tactful way to do it?
There's no denying that getting that Missing Manual™ would be hugely helpful and really speed development time.