Marking for decon is apparently a part of the coming 1.20 release.
This means instead of deconing the reactor you mark it for deconing, at it wont be actually deconned until someone builds it in another spot.
This is very bad indeed. Much worse than the current system. In fact its bad enough for me to create this account to warn about it.
Imagine this:
Griefer marks reactor for deconning, but no-one notice since everything is still working, the reactor is still there.(I imagine a decon mark can only be seen with an equipped construction kit.) Then griefer runs in his BS to some remote part of the map and builds the reactor. PUF the reactor is gone from base, nobody have seen who did it, and nobody knows where the reactor is located. If he build it in front of an alien, the whole alien team will know its gone from base.
On the other hand, if the "marked for decon"-state can be seen by all team members, it might work as intended, IF they can unmark it.