When an instance is shutdown the reverse DNS for the primary IP4 is reset to the default A.B.C.D.vultr.com
However, if an instance has more than one IP, the name isn't reset. I checked, and everything is correct billing-wise, so it's no biggie.
I've been tracking some old IP addresses for a while. When an IP is reused, the default DNS is set then, but in the meantime, you have these lying around :-)
220.127.116.11.in-addr.arpa domain name pointer delete.me.
18.104.22.168.in-addr.arpa domain name pointer deadcat.dns.dyslexicfish.net.
The first one has been dead about a month!
Secondly, a suggestion - when someone attempts to change the reverse DNS, it's worthwhile checking that there is an appropriately matching 'A' record for their entry before accepting the update.
This may help with support calls (typos are effectively weeded out) but is also a bit more secure for Vultr. At the moment, anyone could set their reverse PTR to whitehouse.gov, or more subtley, www.vultr.com
Yes, I know that anyone elsewhere on the internet with control over a PTR could do that, and also that no decent security geek would just rely on un-tested reverse DNS, but I have still seen some apache installs that just store the reverse lookup, and no IP, without verifying with a matching forward 'A', and it would seem to be useful to ensure that your user generated PTRs aren't bogus.
Finally, please let us set NS delegation records for our IPv6 address space to our own DNS servers. I'm not going to anally request it for IPv4 addresses(*), but IPv6, pleaaaaaase!
(*) Contrary to popular opinion, it is perfectly "legally" possible to delegate less than the old-style "class C" without resorting to CNAME hacks. You simply delegate on the /32 instead of the /24