vm.memory.size[used] is currently calculated in Zabbix as total - free. I guess the only reason why it's calculated like that is because how older versions of free were calculating this metric. On newer version of free (version 3.3.0+), used is calculated as it was always supposed to be calculated:

total - free - buffers - cached

The reason why this is better is that you can have an idea how much memory your apps are using, unlike how it's currently displayed used is usually a useless metric. The other reason why it would be good to change how used is calculated is to make it alligned with free, because now when I check Used Memory in Zabbix, and compare it with free it's totally different.

free was changed in version 3.3.0 about 6 years ago and it's used in Red Hat 7.1+ and in Ubuntu for a long time now, so all new distributions now have it. I know that this would be a breaking change, but I don't see anyone making meaningful triggers on this item, even the docs suggest using available for triggers, but would make the item alligned with modern Linux distributions and a lot more useful for graphing.