I've got an old Asus WL-520G running OpenWRT 15.05.1. https://archive.openwrt.org/releases/22.03.5/targets/bcm47xx/legacy/ still provides an image, albeit it is in trx format. Does this mean it cannot be flashed via the web interface? (I tried so and failed). Should it be written with a tftp put?
Not a snowball's chance in hell to boot (4/16). Do yourself a favour and get an actually supported device (urgently), exceeding minimum system requirements (even new device don't need to be expensive, but there are also decent options on the various used markets).
Don't worry, I do have several working devices.
I was astonished to see that there's a 22.03.5 image. What's the reason it is found in the archive when it should never boot?
https://openwrt.org/toh/hwdata/asus/asus_wl-520gu mentions that 19.07.10 was the last officially supported release.
Trying to flash that image via the web interface only yields a "Bad Gateway" error, too, after some moments. I uspect threre's simply not enough memory to perform that operation. Is this the reason why the image is in trx format and needs to be written using tftp?
Not quite, support for devices with 16 MB RAM was officially discontinued with 12.09, more than a decade ago. But even ignoring that, OpenWrt doesn't really proactively remove device support, as long as the firmware image builds (as in, fits into 4 MB), the build system won't notice. It's rare that anyone weeds through the ~1400 supported devices to find those that don't really meet the bar anymore - this has only been done recently, manually, for 23.05.
Depending on your use case, installed packages and adapted expectations, you may find out that your device may no longer be sufficient before its official deprecation date by OpenWrt (which usually trails behind its actual deprecation) - or you may be able to extend its use (dropping packages, webinterface, package managers, etc.) if you get along with reduced functionality, there isn't a clear cut line in the sand (but 16 MB RAM will have a hard time to even boot with the most stripped down image, and finally drop out with OOM once you load the wireless drivers).
OK, points taken. I'm still curious why the images for that model are in trx format and whether that implies that they are not to be applied through the web interface but instead via tftp.
OpenWrt's sysupgrade takes both *.bin and *.trx images (but older versions had a bug in stripping the header, so... it depends… long time ago).