It means they are no longer a forum supplier, so the forum is archived in order to still be visible even though it's not active anymore. As to the reasons why, I'm sure they will vary some (no longer in business, didn't renew their status with ECF, etc).
As for if they can still receive PMs, check their profile. If their account is still active then yes they can still receive PMs, but in many cases like this they've either moved on from ECF or from the industry altogether
There are only a limited number of available spots for "Forum Suppliers" that are given their own sub-forum (and a very long waiting list of vendors vying for that status), so the inactive or no longer registered suppliers on this list are moved out of the way to make room for the next in line