At what voltage are “low voltage” junction boxes not appropriate

High and low voltage conductors in the same junction box must be separated by a barrier. Outdated NEC reference: 800-52(a)(1)c.1.Exception 1.

In the typical dbl. gang box installation, power in one half and phone and data in the other, there needs to be a partition in the box separating the two classes of conductors.

As far as I know junction boxes are ok for closets. Your local jurisdiction may have other ideas. Please consider consulting them.

Today's circuit panels are full of (expensive) AFCI breakers that will be damaged by flooding and running many wires can get expensive with current copper prices. Could you instead run a heavier gauge wire to a subpanel located above the water line and nearer to the loads, and branch out from there?

Also using junctions to "split" a circuit to go two directions should be used sparingly. Most electricians will expect wiring to "daisy chain" from point to point, junctions can make fault tracing much more difficult. Some jurisdictions require that the wire be unbroken from the panel to the first load.