how to use a subscripted variable in a "depends" statement

The simple solution may be to not use subscripted variables.
instead of s[0], s[1], .... use s0, s1.

you can even define  s[n]:=concat(s,n)

............
in addition to Stavros' notes, consider this:

foo(x):= block([a,b], local(a,b), depends(a[x],b[x]))

or variants of it.  Are the properties of symbols lexical?

Also consider  (assume(a>0),  block([a], is (a>0)) )

etc



RJF