I remember reading some topic about this, where it basically came down to how there are so many textures/models/etc included in 3D work that the artists don't actually have the right to use, or haven't made themselves, that it becomes difficult for mods to know what's infringing or not.
I'm not sure if they actually just went and removed anything suspicious because of it - hopefully not - it might simply be that the amount of artists who work with fully original 3D are that rare.
There was also debate about the aspect of making something vs paying for something. For example if you commission something entirely: is it really your own? Do you have a right to post it? The parallel would be buying the models, textures, etc, and making something with that. It would legally be yours, but at the same time it'd be unfair towards those who actually do everything from scratch. In regards to 2D art it's generally not allowed to just trace something someone else has made, for example. Similar to how too many games based upon tutorials, or other games, would be considered shovelware. The balance between right and wrong in terms of legality/technicality feels a bit diffuse.
In the end I think it's still up to the artist at this point. It's still allowed - I think. Though since I don't think most of the people who make 3D art consider this at all, I'm not sure why it's still such a rare form compared to the others. Maybe for that same reason: because there's not much of it here, then 3D artists don't post much of that stuff here either. Maybe the community isn't as interested or supportive as a whole, and if so, a large part may be the stigma that people working with scene creators have brought forth. Like autotune in audio: the term just doesn't conjure the same level of respect as something 'real'. Though there's definitely exceptions.
TheDyingSun
3D art in the portal is what I would call uncommon. It's not a familiar medium to everyone and takes a whole bunch more work for a one time use scene compared to doing a one time illustration. There's also differences in styles and execution of 2D art that is a reason for more 2D art in the portal. I'm pretty sure SFM is allowed as a renderer, but it's in general kind of awkward when you're just using pre-installed models and locations and just pose the character and post that. Being creative on the other hand with using SFM, such as importing your own models and scenes, is better and not so looked down upon.
In summary, don't post a TF2 scene that you just posed the characters. In your case, I think you'd be ok. I assume you designed the scene in the photo above and that's the focus. You just might want to credit the character model.
MOC-Productions
Thanks for the info! Even though I didn't design the original models (which I paid for with a royalty free license), I put a lot of work in to modifying them and creating assets to decorate them with (neon signs, textures, materials, etc) so I feel it's enough to call my own creation where I didn't just copy/paste, then place a camera. I guess I'll post one, then see if I get chastised for it haha.