Yeah… what this likely means is one or both of two things, for this Portal Demake and the Source 2 TF2 thing mentioned by another below:
1: Valve is still quite protective of their IP and may be working on their own new releases of some kind in these IP franchises.
and/or
2: Valve is still quite protective of these IPs and may have identified something like serious misconduct regarding something about these particular projects, or the people working on them… or they just are not looking to be even good quality games, and Valve does not want their actual games to be associated with or confused with games they expect to be of low quality.
I realize option 2 there is a bitter pill for many to swallow, but we are talking about a gaming company that is fairly well known for taking actually good mod ideas and at least attempting to hire or in some capacity work with the devs to create what often turned out to be successful games.
They are notorious for high standards in their own IPs. You’ve got Black Mesa and I think theres one HL2 mod that focuses on you as Commander Shepard from Opposing Force that were both actually greenlit to be sold, for money, as games on Steam, as well as a large number of successful HL2 mods that were not cancelled and are distributed for free by Steam, including Entropy Zero 1/2 and MINERVA.
Its actually pretty uncommon for Valve to DMCA Cease and Desist over mods… theres probably more at play here than just Valve are big meanie heads.