radvd: Option to set DeprecatePrefix to off

[vlcty]

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Is your feature request related to a problem? Please describe.

When I update OPNsense and reboot, radvd sends out a router advertisement setting preferred lifetime to 0 causing all my clients to deprecate the prefix. This is intended behavour, because radvds DeprecatePrefix is set to on in the config.

The config generated by OPNsense:

interface vtnet1 {
	AdvSendAdvert on;
	MinRtrAdvInterval 200;
	MaxRtrAdvInterval 600;
	AdvLinkMTU 1500;
	AdvDefaultPreference high;
	prefix 2001:db8::/64 {
		DeprecatePrefix on;
		AdvOnLink on;
		AdvAutonomous on;
	};
	RDNSS 2001:db8::1/32 {
	};
	DNSSL mydomain.de {
	};
};

Excerpt from the radvd config man page:

      DeprecatePrefix on|off

      Upon shutdown, this option will cause  radvd  to	deprecate  the
      prefix  by  announcing  it  in the radvd shutdown	RA with	a zero
      preferred	lifetime and a valid lifetime slightly greater than  2
      hours. This will encourage end-nodes using this prefix to	depre-
      cate any associated addresses immediately. Note that this	option
      should  only be used when	only one router	is announcing the pre-
      fix onto the link, otherwise end-nodes will deprecate associated
      addresses	 despite  the  prefix  still being valid for preferred
      use.

      See RFC4862, section 5.5.3., "Router Advertisement  Processing",
      part (e).

      Default: off

I understand that this option is on by default and I'd suggest leaving it that way. It makes the switch to a new prefix when using dynamic prefixes painless.
I've got a static prefix from my ISP so my LAN prefix never changes. When OPNsense reboots I loose internet access (that's expected) but I also loose LAN connectivity due to the deprecated prefix. I need to fall back to the link local addresses.

Describe the solution you'd like

I'd like to have a checkbox in the OPNsense Router Advertisements GUI where I can toggle DeprecatePrefix to on or off.

Describe alternatives you've considered

Static IPv6 addresses for everything.

Additional context