could it be windows aero?

nope not on linux

should anyone want to customize these viewmodels themselves, it's possible.

go to yttrium's github where he has the files' source. under the source folder, there are a bunch of .qc files and a bunch of anims folders. save all of these.
the .smd files are the actual animations, and you'll be editing them with your text editor of choice. the .qc files are like a .vmt file, but for animations. the ones yttrium supplies work perfectly.

should you want to get back any animations that yttrium hid, open up tf2_misc_dir.vpk in your tf folder and go to models/weapons/c_models. these are the stock animations in .mdl form. you'll have to decompile them with crowbar to get at the .smd files.

now that you've got all that out of the way, you can start hiding animations. crack open an .smd file with notepad++. before doing anything else, go to settings/preferences and then tab settings. set the size to 2 and check the replace with spaces box. this is to preserve the original formatting. take note of the number of bones in the animation under the nodes section at the top and the number of frames in the animation near the bottom. run this shitty python script, replacing the numbers next to bonecount and ticks with the number of bones and the number of frames respectively. paste the output over all the animation's frames and indent like the original file. you just replaced that animation with a new one that moves the viewmodel offscreen. repeat this process for every .smd animation you want to hide. this is what your .smd file should resemble, varying with the number of frames and bones.

once you're happy with what you've done, you need to compile your .smd files into a .mdl file. luckily, the tools to do so are bundled with tf2. go to steamapps/common/Team Fortress 2/bin and scroll down until you see studiomdl. it's an .exe file that takes .qc files as input. split-screen your c_models folder and the bin folder and drag the .qc files for every class you modified onto studiomdl.exe. it'll send all your .mdl files over to your tf folder, under models. grab those, make folders to match the path models/weapons/c_models as necessary, and drag your new models folder onto vpk.exe in your bin folder. it'll create a .vpk folder with your new animations in it. put that in your custom and, if you followed this guide correctly, it'll work like a charm.