In the Netherlands we've always had coaxial connections as long as I can remember. A quick google search tells me this has been so since the introduction of TV in the 1940's.
This was an adapter for old RF tvs that were mostly in use in the 80s and earlier. They were meant for use with an antenna. By the 90s coax and later RCA (composite) was the standard on most cheaper TVs. You'd probably see S-video or component video on higher end TVs. SCART, on the other hand, was completely unknown in the US.
Kinda weird that having not thought about SCART for years it's come up three times in conversation today. My first taste of 1080p was over SCART. Good times.
They're still a thing, kind of. TV Antennas are still inherently wired this way, and need the part in the picture (a balun) to convert the signal from the "balanced" 300 ohm twin antenna wire to "unbalanced" 75 Ohm coax cable.
Most TVs used to have the twin screw connections to hook directly to a roof antenna. But at some point (I'm guessing the 90s), more people got their TV directly from cable providers, delivered over Coax, so it made more sense for manufacturers to provide a direct coax input for the antenna.
So now, if you do have a roof antenna it probably has the balun integrated right into it, so you can take the coax (hopefully through some lightning protection) directly inside.
300 ohm is only for the folded dipole antennas which have a four times higher impedance than a single dipole. So you don't need to use a balun with a roof antenna, just the right type of dipole.