Most of this, plus USB-serial adapters (I've heard that PCs with native serial ports are now considered "professional" equipment), plus some more exotic ones. A decade ago I used to use port switching boxes and to assemble adapters like Lego bricks to connect the square peg with the round hole . But that's only because I work in embedded.
>>I've heard that PCs with native serial ports are now considered "professional" equipment
I mean, laptops, sure. But PCs I think most still have a COM port. I bought a "gaming" X570 motherboard recently and there's still a COM header on it, I'd need to buy the port itself but the mobo does support it natively.
Microsoft started an initiative in the late 90s to abolish "legacy" ports from PCs including serial, parallel, and PS/2. RS232 is not something you can expect on consumer-grade equipment.
And there's the software too: the buggy Hyperterm (except the private edition), Procomm, Minicom... These days I use Knossos' dterm ( http://www.knossos.net.nz/resources/free-software/dterm/ ).