The DNS Manager has a column "Last Modified"
(when I last changed the slave zone settings).

That's not very helpful.

I'd like to see:

Last Master update (when the DNS last changed)
Serial (of the latest zone file)

I can (and do) check the slaves are up-to-date with dig axfr,
but it'd be easier to glance at the DNS Manager.

Cheers,

Rich/FlatCap

That's not really feasible, since it's possible for you to push different records with a different SOA to each of Linode's 5 nameservers at a different time. There isn't a single centralized timestamp or SOA for your actual zone on Linode's end, because you're in charge of talking to the nameservers yourself.

- Les