@sponsorblock you could download videos in advance, or ask people in the archiving scene if they have an archive like this. Then you could extract the timestamps pretty easy.

Another, and probably more realistic approach is, to get the add timestamps from the innertube api. I know for a fact they can be extracted from the progress-bar renderer and if I recall correctly on another more clean place. Ofc this breaks, if YouTube doesn’t transmit the add timestamps at all. But I highly doubt that will be the case, because they are needed to display the skip button and the colored progressbar. Then the algorythm to get the real timestamps would be very easy.

Since I already develop and maintain a music downloader for which I already reversengeneered youtube, I could reversengeneer it for you if you want to.