Firmware for ZyWALL 2 Plus

Fact is, it is already in production since then, I'm only supposed to take over its maintenance. It's actually doing its job, no reason to replace it (except maybe security wise, but it's only running a small LAN of trusted computers).

I'm pretty sure that it's doing fine: as I sad i have one of that and served as internal router (after the CPE for splitting traffic and avoid nose picking from the ISP) for 4 years from 2016 (which was already out of support and strongly suggested to not use it). I criticize Zyxel but on firewall the hardware is really resilient and reliable.
However if that device is publicly reachable in any way is like driving a car without MOT/Vehicle approval. It works, is doing what's intended to, but when I's time for police checks (certifications) or accidents (security breakouts) might unreasonable the level of hassle. Every device behind that firewall is in heftier and greater risk than with a newer one.
As the one supposed to maintenance that, you're like the car mechanic that make that car run. IMO you should not allow that because it will bring you complicity for some issues. If you maintain an equipment which is unsafe, some laws might consider you co-liable for the fallout.

I'm not in charge for that decision here, but I'll report that suggestion upstream for sure, thanks.

You can still request for dated and signed exclusion of liability if anything happens under your maintenance duty about that device.