It's when you reach Catalina that there's the big break of not being able to run older 32-bit applications (which is why one of my Macs is still on Mojave). Mojave is also a safe incremental upgrade for most users. I believe every Mac which ran Sierra is capable of running High Sierra, and it shouldn't render any older software incompatible. In the short-term, you might consider upgrading to at least High Sierra. The developers have given no indication of a similar end of support for High Sierra or Mojave, but I don't know if they will. When version Quicken Mac 6.0 was being developed, the developers announced several times that that version would require at least High Sierra.

It's always possible - well, it's inevitable - that some future update to Quicken will require a newer OS. But unofficially, it still works on the two operating systems before that, Mojave (10.15) and High Sierra (10.14) I run my Quicken on a computer which is running Mojave and experience no problems. Officially, Quicken says they support the current and two most recent operating systems: so Monterey (12), Big Sur (11), and Catalina (10.15). (It may or may not allow you to download Quicken version 5.18, the 2020 release which was the last to run on Sierra.) Yes, the current Quicken Mac requires macOS High Sierra (10.13) or later.

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