Erasing your Reddit history is an asshole move, and does not really help "the cause"
Users removing their entire Reddit history might block people from reaching useful info that was in that history. Think of people, who may not be Redditors themselves, searching in the future via google, or whatever, for some useful info that was in your deleted history, just because you wanted
to feel good about yourself sticking it to spez, or just because you blindly followed the manic sheeple who advised you to do so.
Deleting your entire Reddit history does not help the cause, not proactively anyway.
What would help the cause is people knowing that a very useful/knowledgeable/interesting user, as evidenced by their Reddit history, has abandoned their platform. Even better, if people knew that the useful/knowledgeable/interesting user has moved to a new platform called Lemmy.
Whether you are planning to leave Reddit completely, or you are splitting your time between here and there, leaving a message like the one below at the end of comments, and/or in its own stickied post in your profile, would be much more helpful than trying to erase your Reddit history:
Bold of you to assume I was more than a lurker on reddit. /s
On a more serious note, although that does MAYBE help spread awareness, it does not stop the fact that reddit would be getting the initial traffic. The one searching for information would be directed to reddit via a Google search, then hear about Lemmy in the comment. This still leads to reddit getting traffic, which is ultimately not the goal. Additionally, because our hypothetical "researcher" is only looking for answers in this case, I doubt they'd be motivated to sign up for Lemmy purely out of gratitude.
I recognize that I'm replying to an old discussion, but I think engagement and staying on the site benefits Reddit much more than hitting a single search result click and getting what you need from it.
IMO (and I claim no particular expertise) if your search results end up with links to edited or deleted posts, you are less likely to do OTHER things on reddit after finding out that's true, because you are still looking for your answer. And eventually, (not suggesting the mass deletions were enough to necessarily cross this threshold) you are going to start skipping reddit results in your searches the way I already skip pinterest and instagram results (though for different reasons).