Are expired patents still prior art?

It might help to remember that this is the whole point of the patent system. Patent publications are publications for the public benefit. It is a one-way street:
- Once the patent application is published it becomes available as prior art. No-one can patent the same technology again. (Assuming the system is woking perfectly!)
- Once the patent has lapsed or exired (assuming a patent was even granted), then the technology becomes free for others to use. (Assuming what you are doing is not caught by other patents still in force!!)

The strategic reason for patents to lapse/expire is the public benefit. This is why maintenance fees are charged: patents WILL lapse unless the owner has a positive interest to keep them in force.