That depends on what video player you use. Of we have control of that, then sure it works. I use mpv to play things, so for radio streams or live videos I can go back/forward as long as it's cached.

But if it's the web service, even though the browser video player has something cached, the player is still controlled by the website. And considering most of the people use chrome/chromium derivatives or YouTube app, it wouldn't be hard for them to make it so that the player itself will collaborate with whatever they want to do.

If YouTube was a separate organization it wouldn't have been the problem it is because of how Google has been taking over all the different parts they need for advertising.