The placement of the M.2 slot on motherboards often seems daft. On my ASUS motherboard, it's directly above my graphics card that dumps 300W worth of heat.
It seems like a lot of video cards are trending towards blower style designs which don't vent hot air inside the case, so if you have one of those it's probably not a big deal.
On my Asus Z270i, the M.2 that's right there has a heatsink too.
Seems very much like the opposite to me, even Nvidia's "Founder's Edition" went to an open design in its latest iteration. Only AMD reference cards seem to still be blowers, AIO designs are pretty much all open.
Now that I look, it does seem like a lot of them are open air cards. I have a mini ITX case, so I only ever look at blower cards since I don't really have the airflow to carry the GPU heat out.
On my Asus Z270i, the M.2 that's right there has a heatsink too.