A question regarding how to manage product updates.

I’m a website developer, I use envato as my go source. I have a couple past clients that I installed plugins that have received updates. What is the best practice for sharing updates? Can the ownership of that product be transferred so I don’t have to continue to monitor a product for someone that is no longer a client? What, in opinions here, is my obligation to a past client?

You have no obligation.
If you bought from your account then they have to get a new license if they want to have it inside their acc.