So for example when I attempted to link based upon routing/account number at Simple, it told me I can't continue because I should hand over my account information for the other bank to Plaid instead.
I've done it, and then immediately changed my account info. So yes, technically Plaid has my historical data, but at least they won't get it going forward. It really sucks though, because it locks my money into a singular bank otherwise.
My understanding of the ACH system is that it's best used in a "pull" manner, as if you're writing a check. Link your Simple account from another bank and initiate the pull from there. (Then work on transitioning your activity to the better bank while you're at it).