Well, it was done in the past, wasn't it. Secondary burials. Votive offerings at springs that may have not been used for a long time. The list probably goes on.

The difference is, as you say, we don't need to leave any other traces of ourselves. We are everywhere already and very, very well recorded in most instances - at least in the 'western world'.

There would be absolutely no difference at all if a local school set up a website that said ... If we were to make a time capsule, this is what we'd put in it.