I see high end equipment, things like RF adapters, attenuators, etc. that have a VSWR specification at DC up to some frequency.
But at DC, you don't have a wave, so how can you have a wave ratio? Perhaps nothing over nothing equals nothing, but the spec is not nothing, it's an actual number.
If no VSWR is possible at DC, there must then be some threshold of where it begins or starts to be measurable. I would guess that as the frequency decreases, VSWR approaches some value until it gets to DC.