That advise was not current I don’t think - adding rel=0 no longer disables related videos, but instead (since 2018) shows related videos from that same channel
From Google
" Note: This parameter is changing on or after September 25, 2018.
Prior to the change, this parameter indicates whether the player should show related videos when playback of the initial video ends.
- If the parameter’s value is set to
1, which is the default value, then the player does show related videos. - If the parameter’s value is set to
0, then the player does not show related videos.
After the change, you will not be able to disable related videos. Instead, if the rel parameter is set to 0 , related videos will come from the same channel as the video that was just played."