On enwiki, and probably a bunch of others, the plainlinks class is used to make external links look similar to internal links, such as for presenting links to a redirect page using redirect=no in the url, or other url parameters such as section=new. Something changed recently, because today I noticed they were being presented in a purple colour in Timeless instead of blue, check out the difference in the shortcut links at the top of the page for WP:VPT with what Timeless is currently showing, compared to that page with Vector and with monobook and with minerva. (I also think the external interwiki links were also somewhat blueish before... don't quite remember)

According to my browser's DOM inspector, there's a css rule

.mw-parser-output a.extiw, .mw-parser-output a.extiw:active,
.mw-parser-output a.external {
    color: #7766bb;
}

that's overriding another rule

.mw-parser-output a.external {
    color: #36b;
}

(both coming from load.php)
Disabling that first rule gives the expected result, that Timeless used to show until recently (as far as I recall).