Examples of pure altruism towards future generations?

One example would be Benjamin Franklin bequeathing $2,000 to Boston and Philadelphia each, which could only be spent after 200 years

Around 1800, five US states passed laws prohibiting the enslavement of future people (at least after they had reached a certain age) [1] [2] [3] [4] [5]. These prohibitions explicitly and exclusively applied to future people (operationalized as people born after X date, where X was shortly after the passage of the law).

More context and caveats:

• I think I read somewhere (currently not finding the source) that these acts of "gradual emancipation" were political compromises between abolitionists and slave owners.
• There were some benefits to present people (e.g., some of these laws also included eventual freedom for some currently living slaves, enslaved to-be-parents would likely get happiness from their kids being better-off, and free laborers might have gotten higher wages due to having less unpaid labor to compete with). But at least at first glance it seems like the primary benefit was to the future people.

Long-time nuclear waste warning messages are intended to deter human intrusion at nuclear waste repositories in the far future, within or above the order of magnitude of 10,000 years.

https://en.wikipedia.org/wiki/Long-time_nuclear_waste_warning_messages

Maybe this? Probably not, though https://www.lesswrong.com/posts/puYfAEJJomeodeSsi/an-observation-of-vavilov-day

Maybe one could find examples of people saving important books in times of severe crises, where it’s clear that the books won‘t help with the rebuilding efforts that will fall on the current generations?

Maybe some construction megaprojects might count, I'm thinking the Notre-Dame Cathedral which took about 100 years to complete. 

This might not really count because the choir was completed after about 20 years. I'm also not sure if it was meant to take so long.

The GitHub Archive Program probably isn't quite what you're looking for, but I think is interesting. It's not historical, and it does have some short-term effects (especially publicity).

Every culture has always been concerned about the future, the afterlife and so on, but it seems to me that worries about "remote" future generations are relatively recent. There are probably isolated counterexamples, though, which I believe are the ones you are looking for. Aside from that, in the animal reign, there is of course the instinctive concern about the "next" generation, which is in turn reproduced in every following generation.

The Aztecs made human sacrifices in order to attempt to avert the end of the world. Depending on what motivations (for instance, status-seeking behavior, perhaps forcible sacrifices, etc) you ascribe to them, this may or may not have been purely altruistic.