Comment 2 for bug 1803203

Andy, in the interest of full disclosure, I'm the other user that reported this through the paid support channel.

Daniel:

The behavior when the preferred lifetime expires is well-defined and handled automatically by the kernel. 'When an address is assigned to an interface it gets the status "preferred", which it holds during its preferred-lifetime. After that lifetime expires the status becomes "deprecated" and no new connections should be made using this address. The address becomes "invalid" after its valid-lifetime also expires'
-- https://en.wikipedia.org/wiki/IPv6_address#Address_lifetime

So on that note, if you're going to support non-zero lifetimes, you should probably also wire in valid_lft at the same time. It would be just a copy-and-paste of the same code for preferred_lft, and they go together conceptually.

To answer your other question, I think if you re-run "netplan apply", it should reset the timer. But take that with a grain of salt, as I do not personally have a use case for non-zero (and non-forever) preferred lifetimes.