file: /proc/sys/net/ipv6/conf/interface/keep_addr_on_down
variable: net.ipv6.conf.interface.keep_addr_on_down
Official reference

Keep all IPv6 addresses on an interface down event. If set static global addresses with no expiration time are not flushed.

0 : enabled 0 : system default <0 : disabled

Default: 0 (addresses are removed)

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.