We can research 
open Standard Notes | F-Droid - Free and Open Source Android App Repository
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Licaon_Kter "standardnotes - disable autoupdates, add nosourcesi...": standardnotes - disable autoupdates, add nosourcesince, disable non-foss builds (ee585f7f) · Commits · F-Droid / Data · GitLab
Scroll down to references @licaon-kter mentioned in issue #3069 (closed) · Sep 5, 2023, 9:11 AM so we go to: Standard Notes License Is Incorrect (#3069) · Issues · F-Droid / Data · GitLab
and now we know.
At the same time… the Note on top is pretty short and on point: “NOTE: Version 3.167.25 is the last one to be free or open-source.”