Depends what you mean by "voting system". Individual voting machines rarely are - they're usually air-gapped, read the ballot from a physical memory card transported by sneakernet from administrative offices, and write vote totals back to those cards for sneakernet transmission to same offices.

However, those central locations use a lot of internet-connected machines, mostly to communicate their numbers to outside systems. The computers that program the ballot descriptions pre-election (a vector for attacking voting machines) are also usually internet-connected - they're plain old desktop workstations of some elections official.

Air-gapping those systems is probably doable, at a minor loss of convenience, but there is a reason they're internet-connected.