Are CSU/DSUs still used for modern equipment?

Solution 1:

External CSU/DSUs have been disappearing, even for T-carrier, for years now, and I haven't seen an external CSU/DSU (connected over X.25 to the router) since about 1999 or so. Routers or interface cards for circuits requiring CSU/DSUs have featured integrated CSU/DSUs since about that time, usually with the configuration of the CSU/DSU moved into software (as opposed to on-screen menu or even dip switches with an external CSU/DSU).

So, external CSU/DSUs have disappeared even for circuits that require them, and circuits that require them have also been replaced by circuits that are terminated with devices that have ethernet interfaces on the customer side.

I'm sure there are plenty of CSU/DSUs left in the world, both internal and external, but since the fastest and most cost effective connections these days do not use them, they are hard to find and their days are numbered.