- The conversion beamer is still available.
- I can also verify it for at least two previous versions and the current 8th Edition rule set.
So I finally had the time to take a look at my books. The link I posted in comments from above is from 8th Edition, given the layout and power rating, though I am unsure what book it is from. Meanwhile, I have found examples of it in two of the previous edition Space Marine Codices as well... The two versions I found it in are the only other previous editions I have prior to 8th. It may have been available in the others as well.
In the fifth edition codex the conversion beamer is detailed under the Master of the Forge:
Conversion Beamer: Incredibly rare pre-Heresy artifacts, conversion beam projectors fire a beam that induces controlled subatomic reaction in the target, converting its mass into energy. The further away the target, the more deadly the blast, as the beam has time to grow in power...
In the sixth edition codex the description is in the section titled Armoury of the Space Marines and is basically the same with the first sentence about its rarity moved to the end; The stats are also the exact same between the two minus the indication that above 72" the shot always misses in sixth edition.
In the eigth edition codex the conversion beamer is not present in the codex, however I did find it in Space Marines Wargear section and under the profiles for Techmarine and Techmarine on Bike of Index: Imperium 1. The Index does not have a description, however the stats for the weapon match those in the image I posted perviously.