some times I have the urgent need to backup a PC that won't boot any more (corrupted system). In the past I used to backup those PCs with a bootable Acronis Backup media. Since I don't use Acronis any more, but mainly backup PCs with Veeam Endpoint Backup, I was wondering if there is an option to backup a PC from a bootable media. As far as I know, the bootable recovery media is only for recovery purposes.
You are correct, Recovery Media is meant for Recovery operations only.
Note that there are tools on Recovery Media that might be helpful in recovering dead OS, like Memory Diagnostics or Windows Startup Recovery.
You should backup machines while they are alive.