MarsSeed [1] filed a deletion request for arduino-samd-core-rc [2]: Abandoned release candidate package from 2020. Since that time, upstream only published newer stable versions but not unstable/rc ones. [a]
[a]: https://github.com/arduino/ArduinoCore-samd/releases [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/arduino-samd-core-rc/
