I'm interested in any historical examples people have of altruistic actions taken primarily to benefit future generations, which don't have strong positive effects for current generations. (So e.g. most climate change mitigation efforts wouldn't count, if we expect climate change to have a big negative impact on people currently alive - but maybe some do?)
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.
Many other religions and cults nowadays try to increase the fertility rate among their members so that in a few generations they will have taken over the politics of the country they’re based in. This may or may not count, depending on how much you’d like to hold them to utilitarianism. Though, note, if a big reason why they do this is because of all the people sent hell for sins, then they are using utilitarianism (despite having a terribly inaccurate world model).