I have never seen an isolation effect as long as a port is present - regardless of the damage. I have attacked, taken a port, and then been cut off by a counterattack. The port had 90+ damage and yet my units were not isolated and were supplied. The latest this happened was in the latest version.
I agree with carlkay58 - if you can trace back to a port, you're going to be in supply if it can trace back to a friendly port regardless of damage. Damage seems to impact the ability to transport/load units not supplies.
IRCC you need 50% to stop supply. But as they repair a few percent each turn you need a bit more than 50% and then keep them there. I have done it. This is the best way to isolate Leningrad for example. But you will never have the bomber strength to effect a port like Odessa. It is only practical against small ports, size 2 or 3 say. Like the ones that feed Leningrad. If he is going all out to defend Odessa he may have maxed out its AA strength as well. Which will make it even harder to bomb.