Here are some options you could consider:
* Decom. (This is a straightforward abbreviation, but may not be universally understood.)
* Dcom. (This is a shorter version, but may be even less recognizable.)
* Dec. (This is an abbreviation for "December," so it could be confusing if used out of context.)
It's best to avoid abbreviations altogether, especially if you're writing for a general audience.
Instead, consider using a more descriptive phrase:
* Out of service
* Retired
* No longer in operation
* Taken out of commission
Ultimately, the best way to express "decommissioned" depends on the context of your writing.