You should include a option in the service plans to add automatically a dedicated IPv6 for that subscription
Why would you want to share IPs with IPv6?
I think this option is a must. Plesk should assign a dedicate IPv6 per domain automatically.
It is definitely time for IPv6 already and it was enabled in Plesk long ago, but still very slowly adopted. This could help.
Javier Narváez
shared this idea
Unfortunately, we have to close your request, because over the years it has not become quite popular for further implementation.
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Tozz
commented
The reason you want to use shared IPv6 IP's is because you want to prevent your routing having to keep up with an enormous neighbor table (IPv6 ARP equivalent) if you have a dedicated IP per website. If you would, eg. host 25.000 websites your router will have to perform neighbour sollicitation for 25.000 IP addresses. This will exhaust your router's neighbor table, causing traffic to get broadcasted. Keep in mind, 25.000 IP's is equivalent to almost 100 IPv4 C-classes.
A way to fix this is to route a subnet (eg a /64) to your Plesk server, but I doubt you do this as of this moment. If might fix your neighbor table issue, but it increases your router's configuration, which is also bad and difficult to maintain if you have a lot of webservers.
So in the end there are good technical reasons why you want to maintain hosting multiple sites on single IP addresses.