tl;dr My experience with posting solutions is that it actually makes them less effective as study tools, therefore I don’t do it.

Posting past exams is always a dilemma for instructors.

If you don’t do it, then students that are part of some “organizations” that collect past exams and make them available to their members have an unfair advantage over others.

But if you do post them, then students begin to ask for answer keys, and become frustrated if you say no.

Full disclosure: part of it is a matter of time.
Posting a collection of old exams takes about 15 minutes.
Constructing a usable answer key for all of those past exams can take several hours.

(You might assume that there is a usable answer key sitting on my hard drive that I’m just stubbornly not sharing with you.
That’s not actually how this works. Questions often don’t have one single “right answer”, and that’s why the answer key that a grader uses to grade isn’t necessarily publishable as an answer key that students could use to study from. Publishing the answer key that a grader grades from could potentially lead to even more confusion than not releasing it at all.

But that’s still not the central issue, so let’s just set that aside for now.)

I do many things for my class that take many hours—if I thought that putting up a comprehensive answer keys was important to student success, I would spend my time there, instead of on other things (such as learning about test coverage, or setting up an elaborate system for doing legacy code projects.) In fact, I used to do it.

I chose* to stop doing it because my experience is that **when I post answer keys, the old exams actually become less effective as study tools.

Contrary to what you might assume, the old exams are more effective as study tools without answer keys.

That will clearly seem counterintuitive, so here is a more detailed explanation.

What happens when I don’t post answer keys.

Students have to treat the past exams as authentic practice questions
If it is a programming question, they have to check their answers by actually typing in the code, and seeing whether it compiles or not, and does the right thing.
If it is a vocabulary question, students have to check their answers by looking up the answers in the textbook, and re-reading the appropriate passages. Reading those passages results in deeper learning.
Students are also free to bring their completed sample exams to open lab time or to instructor office hours and ask questions there. Those interactions lead to deeper understanding.

What happens when I post answer keys.

Some students actually authentically do all of the same things that I listed above for what happens when I post answer keys.
I’m going to give students that ask for answer keys the benefit of the doubt, and simply assume that intend to take exactly that route—i.e. the diligent scholarly route.

But as it turns out, that’s not what most students do when offered an answer key.
Instead, students use it as a “short cut”.
To be perfectly honest, I would probably be among those students, not because I’m lazy, but because I tended to be overcommitted (and still have that tendency)—I’m very curious about many things, and want to take “too many classes” and take on “too many projects”. So, not out of laziness, but out of “survival”, I look for places to cut corners.

And sadly, that results in some unfortunate behaviours. This is what happens when there are answer keys:

Fewer students actually attempt the problems. They read the problem, then the read the answer. Very little learning takes place, if any at all.
Some students read the problem and then immediately read the answer. Overconfident in their abilities, they think that they “could have” solved the problem. But it is wishful thinking. They go into the exam less prepared than they need to be. Faced with a problem without an answer key, they are stumped.
Some students just copy the old exams and the answer keys onto their notes sheets in the hope that I’ll reuse an old question unmodified. When they see that I’ve slightly modified the question, they try to reverse engineer how to slightly modify the correct answer from their notes sheet, rather than actually understanding how to solve the problem. This results in virtually no understanding of the problem—just an attempt to get as much partial credit as possible with as little effort, and as little learning as possible.

For more exploration

Some background on how various kinds of students approach a curriculum, and how that guides my teaching practice, can be found in this short 19 minute film. If you want to know what guides my choices as an instructor, the ideas in this 19 minute short film (available in three parts on YouTube) is pretty much the foundation that guides all of my choices.

Teaching Teaching and Understanding Understanding (video)