I don't know about open source ones, but I did find Enable Banking (https://old.reddit.com/r/eupersonalfinance/comments/k4ny3j/f...) offering free access to your own accounts. I don't know if "offer" still stands, the FAQ is not very explicit on it (it only talks about testing, https://enablebanking.com/docs/faq/#can-i-test-the-api-befor... & https://enablebanking.com/docs/api/linked-accounts/).

I tried it, but I couldn't get things working sandbox environment so I ended up giving up and just do manual exports.