And even now both iOS and macOS warn you before you update your reminders or notes if you have other devices not compatible with the new format. Nothing stopping anyone from just continuing to use the old format of reminders or notes.Well there COULD be but that’s not a necessary thing to happen. And even then you could likely use an alternative rather than purchasing a new computer right away.
Apple generally does quite a lot to make sure devices remain usable/compatible even on older OS versions.