These statements sound more like urban myths to me. Adding salt to a steak that is about to be sous vide will definitely not have a significant effect on its texture (we're talking normal amounts of regular salt, just for seasoning purposes and we're also not talking about curing salts which would have a much harsher effect). Salt does affect protein and cures it (denatures it) which in fact tenderizes it. So, if anything, adding salt might help your case but I can tell you the difference will be well... negligible.
Salt does draw moisture out of meat (which might be why some people could be concerned with meat becoming tougher...) . First, the amount of water drawn by seasoning a steak with salt is close to nothing really... and over time, that little moisture will be re-absorbed (the proper term would be "diffused back into the meat") by the meat if you let it sit or add it to the SV baggie. The same principles working when brining meat apply to dry rubbing meat with salt. So go ahead and season your meat before bagging it.
And regarding garlic, onion... harshness. Use whatever you like. Garlic is harsh cooked sous vide at low temperature because that's what garlic is supposed to do. That harshness will go away once you sear the steak. If you pre-sear, then sure... the steak might turn out garlicky but hey... I'm all for that.
If you're interested in the effects of salt on food, check out this simple test on my blog:
https://thatothercookingblog.com/2013/06/17/how-does-salt-affect-eggs-in-cooking-had-enough-gotta-test-it-myself/