You can user Power Delete Suite for this purpose; that's what I did. It missed some comments/posts, but it got enough that I could go through and manually edit the stragglers.

For those who just want to edit comments to random letters and then delete, redact also works (but requires login) and might be more thorough.

Idk really if my copypasta about deleting my comments/posts in protest of the reddit api changes, and about lemmy, will do any good (I suspect the account may just get quickly banned), buuuuut it sure did feel good to do. I'm glad to no longer be supporting that website in any way, shape, or form. Although I understand the people who think it's better to leave their comments/posts up for the other users, too - that's valid also.