I’m still on 21.02.5, basically because “if it ain’t broke, don’t fix it” and I haven’t seen any significant security advisories at https://openwrt.org/advisory/start
My setup is simple (home network with 500/100 fiber to ISP, WiFi main + guest, ordinary clients - laptops/phones, SQM).
Overall I don’t have problems.
The only issue I have is occasional dropouts in Zoom because of a very noisy neighbor. That’s mostly mitigated by setting the 5GHz to 40MHz channel width.
Since I saw 25.12 has just been released, I’m asking myself if I have anything to gain by upgrading?
If there are no security issues and the current configuration works well, there's no immediate benefit to upgrading.
If you want to improve WiFi compatibility and have the latest patches, updating to 25.12 is recommended, especially as a future prevention measure (remember, version "21.02.5" is "end of life").
However, you should back up your configuration and consider the possibility of having to reconfigure some settings after the upgrade.
You can not do step-at-once upgrade till the end, backup config, upgrade to 25.12.0
There will be breakage down the road between v23 and v24 due to switch architecture change from swconfig to DSA (that needs re-entering switch config manually)
Firewall offload is broken on mwl in v22/v23 (disable it beforehand if you plan rstoring partial configs)
If you have custom firewall scripts (like firewall.user with iptables commands) - suggest playing them out in a 25.12.0 virtual machine with 2 netcards first, fw4 has different includes style.
Maybe, but . . . Why did you opt to use OpenWrt in the first place? A lot has changed between 21.02.5 when the WRT1900ACS moved to DSA from switch arch and other noticeable improvements through to 23.05.6.
To answer your ?, I would.
You get the security of knowing your device has the latest security measures that could affect your device.
You get platform specific accumulated improvements since 21.02.5
Better support when you need it. Folks seldom remember changes several Releases removed.
With a dual partition device you always have a fallback to 21.02.5 if you want to revert.
Take a backup, flash current Release using a Factory image don’t keep any configs (use the backup to help configuring the newest release).
Like @RuralRoots implies, that doesn't mean there aren't any. No one goes through and documents the kernel patches that address vulnerabilities (I'd guess many thousands between your 5.4 kernel and current 6.12), they just assume you'll stay up-to-date with the latest version...
Additionally, 21.02 has not received the benefit of any upstream patches since April 2023, making the situation even more dire.
Except that it is known-broken in regards to security issues (and there will be many more than your casual 30'000 ft overview from https://openwrt.org/advisory/start will give you).
@RuralRoots you said flash with the factory image (I see in other posts you recommended this because the kernel size can change between releases). Is it OK to do this from the UI or must it be from the CLI? If so, what options to use?
After flashing the factory image on the “other” partition, I’ll still have 21.02.05 on the current partition. So for the next upgrade, should flash I that partition with the factory image too?
Yes, you can do it from the LuCi GUI. Uncheck all options presented.
Make sure you make a backup for reference. You will find it handy in helping you configure your system. FWIW, most of your backup other than Network and System can be used verbatim.
No. You can go back to following a sysupgrade process of your choice. I only recommend using a factory image when making multi-version jumps. It just makes sure that all device changes get updated. Anyone upgrading from Release to next Release version should just continue to use sysupgrade. Judicious use of a Factory image won’t hurt anything only on this specific series of the Linksys routers.
Just to be clear for others potentially watching from the sidelines, the factory images being compatible with sysupgrade is a specialty of these mvebu based Linksys wrtxxxx routers, it's not generally the case for other routers from other vendors (where doing this would be harmful).
Just for my understanding: cat /proc/mtd on my 21.02 install shows the kernel partition as 6 MiB, the same as that shown on the wiki page https://openwrt.org/toh/linksys/wrt1900acs
So my the kernel partition size is currently already the same as the 25.12 build expects.
So is there any other benefit to using the factory image?
All I did was to verify that on the WRT1200AC, WRT1900AC V1/V2, WRT1900ACS V1/V2, WRT3200ACM, and WRT32X devices flashing a factory image would not be detrimental to the device functions, and ensure any device mods between then and now would be incorporated.
Conventional advice for OpenWrt when upgrading over several Release versions is to sysupgrade to each release in turn. To your initial query I said that you could try a current Release and go back to your 21.02.5 if you wanted to go back. If you follow the conventional advise you cannot easily return to it.
No. There was a bug workaround to disable tx amsdu on mwlwifi that IIRC, was fixed on #12500, and then there was a huge mwlwifi update thanks to @jbsky on #14044 to fully fix amsdu, wpa3 and many other improvements.