Yashin is ancient history at this point so it is difficult to learn anything from that trade that we can apply to a hypothetical trade that happens this offseason.
Players with Eichel's ability are almost never traded at his age. Hall (1st trade) and Seguin are the two that have happened in the last decade. Neither at the time of the trade were as good as Eichel is now. Both had supposed issues, where as Eichel just seems to be upset with losing. There aren't really any indications that he's involved in any drama otherwise.
Boston also made the trade partially trying to 'win now'. They got a star 1st liner who was cost controlled for a few more seasons. Buffalo isn't in that same situation.
Usually when young guys are traded, it's a 1 for 1 type of trade where teams are placing a heavy emphasis on team need, Hall for Larsson, Jones for Johansen, Drouin for Sergachev, etc. Hall for Larson was positional/cap need for EDM, and Jones for Johansen was positional for both teams. Buffalo does not have that same motivation since moving Eichel would destroy them down the middle.
My off the wall idea in the main boad thread, which is sure to get me in trouble, would be if Toronto loses again in the first round to Boston (even if they don't play Boston, they'll find a way to lose to them), Matthews for Eichel. This is assuming there are actual issues with Eichel and Buffalo. A lot of people are going to say Matthews is better, which might be the case, but with the cap possibly being stuck at 81.5M going forward, that kind of move would help the Leafs manage the situation better. Not because they clear a bit over 1 million per season, but because Eichel is cost controlled for longer than Matthews. He has an extra two years of term. Judging by how aggressive Matthews was in the original negotiation, he is going to be looking for a huge UFA contract 4 seasons from now.
The biggest benefit of the swap is that both sides help each other out, because the trade would void Matthews' and Eichel's future NMCs. Both sides would obviously hope to keep their respective players beyond their current contracts, but that would give them even more flexibility going forward if that turns out to not be possible.