This is very unfortunate and surprising, goes to show that even after 14 years Flattr didn't find any market. I was hoping for bitcoin and crypto to show low usage after around 15 years of no legitimate use cases other than speculation, ransomware and other scammy things, but Flattr's shutdown was a surprise. Flattr billed itself as the RSS of donations to really get rid of those ugly PayPal buttons on blogs, website…
Their idea of accumulating microtransactions ("Flattrs") internally for a month and dividing up a fixed amount was a really nice idea: both to reduce payment processor fees, and to remove any anxiety about clicking on a Flattr button (the same amount was spent per month, regardless of how many things were clicked, so go ahead).
However, those monthly payments were also internal: transferred between from one user's "account balance" to another, and those balances had to be topped up manually via a rather laborious bank transfer (supposedly to keep fees down). I think there was an assumption that money would be moving around a lot inside the platform, e.g. blogger A gets Flattred for their post, and they Flattr developer B's application, whilst B Flattrs musician C's song, etc. I think reality had more of a creator/consumer dynamic, which made it more important to smoothly get money into the platform.
I think they would have fared better by allowing users to be automatically charged each month (like Patreon, etc. who came later). The "account balance" idea could be simplified to withdraw-only, for tracking how much was received (i.e. 0 for almost all users); and perhaps allow that balance to be deducted from the monthly charge. It seems obvious in hindsight, and perhaps they did change to such a model; alas, I think their early buzz was squandered due to this friction.