devtrackers.gg
This was talked about on Q&A a bunch a while back. They said it would be nearly impossible to gate content based on time of release.
Neat idea don't think its even possible
Definitely not "impossible".
I guess it depends what you determine as content. Locking key pieces of content would be much simpler than bringing back unidentified herbs for example or disabling a bunch of QoL stuff we've done over the past 7/8 years. A good example of this is we could unlock Chambers of Xeric as part of 2017 but it would be the Chambers of Xeric you know now and not the version we had back on launch. So in short, we could do it with only major pieces of content and it might work?
I think to me the idea needs something a bit more to entice people into trying out the league. Perhaps a way to gamify the unlocking of years helps that similar to what OP suggested where it's determine on tasks completed instead of unlocked week by week?
Edit: Just wanted to say it's an awesome concept though and these are the kinds of things that I'd love to see the community come up with for the next League. I think Trailblazer really opened up what a League could be for many players outside of just "area-locked to a single region"
Isn't the main issue that it would be time consuming adding a property to every piece of content to include the date it was added?
Not necessarily. It wouldn't be too hard to add the check to every piece of major content. I think part of the design would be evaluating all the content in the game and deciding what we can and can't lock off and then only doing those ones and if anything is too complicated we leave it as is