My question is simple: in which situations would OpenWRT hit the maximum 27 MB RAM available in the system ? I ask this because the articles state that the memory limitation is worse than the storage limitation, but I can't see this limitation while using the router (and this invalidates the articles arguments).
Typically operations that cause memory peaks due to some large file handling/parsing. E.g. Adblock block list handling, opkg package list handling, etc.
You are already doing the main mitigation by running an ancient deprecated OpenWrt version, which has smaller memory footprint due to the old kernel. That is of course unsecure, but enables you to use the router.
Hello, does this mean that during normal operation will the router stay far below the memory limit and only during specific procedures it will exhaust memory ?
Do you know how much that 14 MB memory usage would jump to by upgrading OpenWRT 18 to the latest version ?
Overall, it seems to me that this version is completely usable and the arguments described in the warning pages do not match practical behavior of OpenWRT while being used in "cruise mode".
Depends a lot on your use case. Which apps/services are running etc. Minimal-feature fixed-line router survives more easily than an adblock-enabled heavy-wifi-use router.
You might test it. You can always revert back to 18.06 if 19.07 does not work for you.
Latest version, 21.02, would be too large, pretty sure about that.