Editing for clarity.
If by decommission you mean "stop paying the support fees, but keep the software running",
Assuming what you intend to do is to continue using the software but stop paying the support & maintenance fee, that's exactly what will happen. Your current version will continue running, and everything will work as is, but:
- you won't be eligible for updates, upgrades or hotfixes (actually not 100% sure on hotfixes, as they could potentially include security fixes - I would ask support to be sure)
- you won't be entitled to open incidents with support
Normally the on-premise licenses for Tridion are "perpetual", you have the right to run the software forever (there are exceptions to this, not all licenses are the same, and you should check your contract).