You can add "cards", i.e. the popups in the top right that show a short text and an "i" button. Those can be links to videos, polls, or external links (if you're a big youtuber). However the text is limited, you can't set how long it shows, and you can have a maximum of 5. You can maybe use subtitles too, but you cannot set a default subtitle for your video.

So there are hacky half-working workarounds, but otherwise youtube just told everyone who cares about their videos to go and kindly get fucked.