Delayed insulation failure of DC barrel jack
It's not insulation failure that will be the first problem, it will be arcing between contacts. Most insulation is rated for 100's of volts. But really it's the conductor spacing that can be a problem.
Below is at table of IEC regulations. For 100V the spacing needs to be 0.2mm between any conductor. So between the plus and minus terminals of the DC barrel jack the distance needs to be specified for the greatest voltage. So if you think you need 1000V then make sure the spacing is right.

Source: https://go.gale.com/ps/i.do?p=AONE&amp;u=googlescholar&amp;id=GALE|A111490273&amp;v=2.1&amp;it=r&amp;sid=AONE&amp;asid=81d5a14e
Secondly the DC barrel jack should have a datasheet and it should have the highest voltage allowed, so that can be checked also, make sure the jack will be specified for the greatest voltage.
Insulation does breakdown, but it should show the rating on the insulation of the cable and if you use a jack it should have a voltage rating which it (should have) been tested to.