Season 1 had SR on a scale from 1-100. Seasons 2-7 were similar to the current system and on the current scale, but win/loss streaks were much stronger then, and possibly (likely) the performance modifiers were stronger too. (I don’t have a reference handy, but I believe that with or shortly after the removal of PBSR from Diamond+, the effect of PBSR was softened in other ranks). Due to the dampening of the streak effect (and maybe the PBSR effect), you cannot get as dramatic of gains now as you could then.
Season 2 in particular I believe would have had a full MMR reset due to the rework. It’s unclear to me which seasons had the partial MMR resets that Scott Mercer mentioned, but I assume they were early ones. If your MMR is reset, then your placement ‘uncertainty’ goes up, giving you higher MMR and SR gains and losses. You can also make ‘uncertainty’ go up by not playing for a long time, but as as long as you are playing regularly the system will have a high confidence in your placement and give you standard gains and losses.
I believe a smurf on a new account can still see gains in the 50ish range, although I haven’t watched one of those videos in awhile. I think it would be extraordinarily difficult for an ordinary player to do the same.
PS: Most of what we know about the MMR/SR system is in this post, or can be found from the links in the references section of it: