Do you have two NICs (eth0 and eth1)?

Yes

I take this as a solution, and will remove the portion which propagates the ipv6 range to the subnet
I felt, this was necessary, but is not in view of the existence of private subnets

I used this advice
https://subnettingpractice.com/how-to-subnet-ipv6.html

BTW the idea of having private ipv6 subnets is and contrary to the original Idea of worldwide ipv6 numbers.

Thank you

You can do the same thing even if you only have one physical device, with a virtual interface (for example, ifconfig eth0:1 64:ff9b:1::1).  I do this with IPv4 addresses and postfix.

AFAIK, there's currently no other way to configure postfix for dynamic IP addresses, except for inet_interfaces = all.  I could be wrong there, but from skimming through http://www.postfix.org/postconf.5.html I'm not seeing it.

These statements appear to be contrary to each other.

No contradiction within
the ISP provides a prefix range /56 or /64, of which a portion is assigned through propagation into the subnets behind.
e.g. when I unplug the WWW router and reconnect it, my subnet has new ipv6 numbers.

I still want postfix to listen only on the LAN and localhost for local messages. So postfix must know the actual IPv6 numbers of the LAN interface

netstat -tulpen | egrep "master|PID" 
Proto Recv-Q Send-Q Local Address           Foreign Address         State       Benutzer   Inode      PID/Program name    
tcp        0      0 127.0.0.1:465           0.0.0.0:*               LISTEN      0          6565139    20047/master        
tcp6       0      0 ::1:465                 :::*                    LISTEN      0          6565140    20047/master  

on Ipv4 postfix listens only locally, on ipv6 postfix listens on all interfaces.
anyone knowing the ipv6 address may try to use postmaster (fortunately behind a router blocking the port)

Postfix shall listen on eth1,(LAN) but not on eth0 (World).

These statements appear to be contrary to each other.  Do you want postfix for local network only, or for world (internet)?  If you only need it for LAN, then it does not matter what your ISP assigns; just set up a static local address to use on eth1.
If you actually do want it to be reachable from outside your network (internet), then is there any reason not to use inet_interfaces = all?

/etc/postfix/main.cf:
    inet_interfaces = all
    inet_interfaces = all (DEFAULT)
    inet_interfaces = 127.0.0.1
    inet_interfaces = 127.0.0.1, [::1] (Postfix version 2.2 and later)
    inet_interfaces = 192.168.1.2, 127.0.0.1
    inet_interfaces = localhost

Postfix shall listen on eth1,(LAN) but not on eth0 (World).

what is missing here:

 inet_interfaces = eth0 
 inet_interfaces = eth1

with ipv6 I do not have fixed ip numbers, because these will change dynamically

Starting Postfix Mail Transport Agent: postfixpostmulti: fatal: config variable inet_interfaces: host not found: eth1

The ipv6 numbers will change dynamically , what ever I receive from my ISP .
They (dhcpv6) might last 3 months or 1 hour. Both I had already.

How do I have postfix bind to an interface such as eth1 without mentioning the ipv6 numbers ?