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

If enabled, do not classify optimistic addresses as deprecated during source address selection. Preferred addresses will still be chosen before optimistic addresses, subject to other ranking in the source address selection algorithm. 0: disabled (default) 1: enabled

This will be enabled if at least one of conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.

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