Every since I started aggressively overriding websites so I could see what links I'd visited, it's become really clear to me that modern web design apparently hates making visited links look different and goes out of its way to not do so. This is most visible on the quite a lot of text focused websites that still stick to more or less the original web colours of black text on a white background with blue links. In the original basic web colour scheme, visited links would be a purple shade. But although all of these websites stick to the basic black, white, and blue colours, they wipe away the purple of visited links.

Some websites opt to style their links in some way other than with colours (often underlines), and for those websites not styling visited links differently is understandable (even though I disagree with it). These sites have decided to have text and text decorations in a single colour, and you can't significantly restyle visited links these days (for good reasons). So, for example, if you use a solid underline for unvisited links, you can't use a dashed one for visited ones (as far as I know). Other websites opt to significantly change some or many of the colours for whatever reasons, so not styling visited links differently avoids coming up with an additional colour (and colours are hard if you take it seriously).

(Also, if I'm understanding CSS right, if you set a colour for <a> elements it applies to them in all states. Once you start setting link colours, you have to go out of your way to colour visited, active, focused, or hovered links different than standard (unvisited) ones.)

However, quite a lot of text focused sites stick to the basic colours for text content but more or less deliberately wipe out any special colour for visited links. These sites could easily continue to let visited links be as they are when unstyled, but they don't. Whether this is done deliberately or is simply casually accepted, it's clearly part of today's web design aesthetic (and probably has been for a while).

I'm sure I noticed this subconsciously before, but actually creating site style after site style in Stylus has rubbed my nose in just how many of the sites I wanted to fix use the standard black, white, and blue colour scheme. It's also made me aware of how common a basic scheme of black, white, and underlined links is (it's probably the second most common one I alter).