I've watched a lot of videos with newspaper or magazine clippings or even short snippets of TV ads and the like, and I'm pretty sure they haven't gone to each one and asked permission first and are probably relying on the fair use defence. Would that be likely?
Absolutely. If you're using reference material to illustrate or expand on a point you're making, that's exactly what the (USA's) Fair Use clause was created to defend. It is in the strongest spirit of the law, and no company would waste their time spending $4000 for their legal department to draft a cease and desist letter for usage like that. Legal doesn't lift a finger unless they think your usage is causing the company financial harm.
People get on trouble on YouTube for using clips of other video or audio that are "active primary consumer product" (music, movies, TV shows, etc. that companies are actively defending). It is very difficult to claim that as Fair Use unless the usage is commentary on the item itself, but YouTube sides on the content creators and has lots of AI and tools to help flag inappropriate usage. YouTube doesn't decide Fair Use -- only a court of law can do that -- but they can, and do, err on the "safe" side which is the original content owner's side.
Bottom line: If you're using an old commercial, or a scanned catalog or manual for a vintage computing subject, you have nothing to worry about.
If you need a case study, I have both scanned manuals and full commercials in my AT&T 6300 Retrospective video, and that's been up for 7 years without a single complaint, flag, strike, takedown, etc.
I do wonder why the people producing it often post their wares with no credit requirement.
Because either they released it to the Public Domain prior to YouTube, or YouTube negotiated a one-time non-exclusive license for the privilege.