I completely understand the frustration, and I’m sorry the 26+ requirement is a blocker for some of you.
The short version: Lettera is built around the new macOS design language, and supporting older versions would have meant a lot of extra work for relatively little return. We’d rather put that effort into making the app great on the platform it targets than spread ourselves thin maintaining compatibility layers for older systems.
It’s not a decision we took lightly, and I know it’s not the answer some of you were hoping for. We’re keeping an eye on the feedback, so thank you for letting us know it matters to you.