[Aachen]

The oldest instance of it that I know is putting something like download before or after the youtube domain. This must have been 2008±2. I very much doubt that's the first instance ever but I wasn't around online in the 90s (aside from on my grandma's machine who didn't know her computer had a web browser, but that wasn't very conscious, just a neutral net (young me) clicking buttons to see the effect)

[theolivenbaum]

ss before youtube.com brings you to a download page (i.e. ssyoutubecom)

[aembleton]

Subject: Discontinuation of Service in the Great Britain

Blocked in the UK :(

[KomoD]

or youtube5s