I'm struggling to think of a part of the OS that hasn't had ads shoved into it... the terminal I guess... There have been ads in the start menu, the lock screen, in pop up notices, in the file explorer, in search results, in the control panel, on the task bar, in the share pane, in windows update and in a bunch of windows apps like ink workspace. They've even just force-installed random programs to people's systems.
I'd wager, too, that the addition of the garbage you're describing has coincided with the OS's worsening performance. File Explorer performance is so abysmal that it may as well be an Electron app.
On the other hand (edit: regarding your first paragraph), Microsoft seems very serious about not falling afoul of the law, probably because of the cost of the anti-trust litigation they faced in the 90s and 2000s(?). It wouldn't surprise me at all if there were nothing for a whistleblower to blow the whistle on.
That's the definition of monopolistic behaviour that almost got them split up 25 years ago.
Apple was selling a desktop operating system that was a competitor to Microsoft Windows, Corel WordPerfect was a thing and so was Lotus 1-2-3.
Apple also had a much smaller desktop OS market share at the time. They nearly disappeared but are in a much stronger position today, which makes it harder to argue that Microsoft has a monopoly. There's no strict threshold for market share in these cases, but it's one of the factors taken into account.
Yeah, they do way worse shenanigans now.