When did URLs first appear in matrices?

Can an approximate start date be determined for the first use of such a marking, similar to the first use of SID codes in 1993?

Honestly, I don't think so. One could argue about the timing of the proliferation of the Web, but that won't mean much imo. Nothing can obviously be exact, and it's case-by-case for manufacturers anyway I feel.

Best you could do is check to see when certain domains were first ed. Anything with a www. on it isn't going to predate April 93, when CERN opened the web protocol open to the public, probably not even predating 1995 at all, as I don't think the WWW prefix was around until after the W3C was creating in late 1994.