Is the 970 PRO still worth buying?

Yes if you write large files often and want steady speed and high endurance. If you mostly game or browse, a cheaper PCIe 4.0 TLC drive gives more speed for less.

How is it different from the 970 EVO Plus?

The PRO uses MLC NAND and holds write speed better during long transfers. The EVO Plus uses TLC and is cheaper, with a lower endurance rating.

Will it work in a PCIe 4.0 or 5.0 slot?

Yes. It is backward and forward compatible, but it will run at PCIe 3.0 speed, so you will not get the full bandwidth of a newer slot.

Does it come with a heatsink?

No. It is a bare 2280 M.2 drive. Most desktop boards and laptops cool it fine, but heavy sustained writes may benefit from a heatsink.

The drive itself and documentation. Mounting screws are not included, so check whether your board or laptop provides them.

It is fine for an OS and everyday apps. Large video projects and game libraries will fill it quickly, so consider a larger capacity if you store big files.