Why do modern routers not require clock settings anymore?

The whole dce/dte thing is still correct, in theory. However, today's devices generally insulate you for those details.

I bet your ds3 cable connects directly to your router, right?

It used to be that the only way to connect to a router was a serial cable. Slightly more modern routers got ethernet. It was a long time before anything else was available. If your connection wasn't a serial cable or ethernet then you needed a csu/dsu to convert it to serial.

With Ethernet you had standardized signaling. For the most part, just plugging in an Ethernet cable was all you needed to do to "configure" the physical layer.

Serial cables, on the other hand, provide no information about the timing characteristics of the signals on them. Since the serial port had to support a bunch of different timing possibilities to be able to connect to a variety of different interface types, and the serial cable didn't provide any hints as to the signaling format, configuration was needed in the router so it knew what to expect coming from the csu/dsu.

Now fast forward a bit more and you start seeing routers with built-in ISDN and t1 ports. These connection methods carry with them more assumed parameters (such as the clock rate and signaling format) so those options disappear. For example, a t1 has a fixed bit rate, so that option disappears. And since the csu/dsu is built in to the router, all of the options about cable distance and equalization disappear.

So this directly applies to your ds3. There is only one clock rate possible on a ds3, so you don't need that. Is the ds3 clock provided by the telco? If so, then you don't need any clock settings because the router will detect the clock and latch on to it.

In fact, the bandwidth number is probably unnecessary. First of all, that number doesn't actually limit the bandwidth of the port. It is used by other processes in the router, such as routing and qos, to make flow decisions. What is it set at? If its the full bandwidth of a ds3 then it's unnecessary.

Tl;dr: today's routers accept wan connections directly instead of going through a csu/dsu to convert it to a serial cable. The interface type (isdn, ATM, t1, ds3, oc1, etc.) implies a lot of things, such as clocking, and therefore do not need to be configured. Your study materials are teaching you about converting things to serial first, which is becoming more and more rare these days.