What happens if I don't upgrade Mac oS or iOS?

Hi Stephen. Updating to latest and greatest OS available is not necessary if you don't want to. It's bit hard to say how long Big Sur is going to be supported as Apple doesn't publish a support roadmaps. But I expect it to be supported till late 2023, as older versions like Sierra that was released in 2016, and was supported to late 2019 with security updates. That's just for security/combo/feature updates, 3rd party programs will be updated for longer period of time usually, but those differ per program.

I can tell you that even on unsupported/obsolete version of macOS, the computer works just like it did should. Websites will open and features are still there but your OS will not receive latest security patches. I had High Sierra on my Macbook Pro that's classified as obsolete by Apple and it worked just fine.

Staying on current OS (Big Sur) for multiple years is completely viable option. Assistive features like voiceover being buggy is not a good thing at all, and if it works fine now, postpone the macOS update as long as you can until bugs are ironed out.

If you want to prevent your MacOS version from updating, you can go to "System Settings - Software Update - Advanced - and turn off "Install MacOS updates". That way you can monitor what kind of updates are available, like is it security update or full system update.

To the iPhones. Apple maintains a list of Supported iPhone models at support.apple.com/en-in/guide/iphone/iphe3fa5df43/ios if you want to take a look. I've seen that iPhones will be supported up to 5 years after a particular model is no longer in production or even longer. So as example my iPhone 7 was discontinued in 2019, so it should be supported till 2024

Your iPhone XR is still in production, so it's going to be supported for a long time now on.

I hope this helps :)

Anything that currently works will continue to work.

Apple doesn’t change the old OS versions except for Security updates which do not add features, but could theoretically render something unusable, but I can’t remember that happening.

The only things that would drop support would be newer OS versions.

If the websites change to using some new technology not currently supported, that could also be a problem, but not likely.

As in any Newer Version of macOS - the initial release will have bugs which normally are addressed in subsequent updates.

Staying on present versions of macOS and iOS - that work for your special needs - is an idea. Understand though, Support for Big Sur will eventually end at some time in the future and only Apple knows the. Normally, an OS gets about 3 years of support once it is released. So Big Sur released in November 2020 would in theory - continue until maybe 2023. The OS will still function just fine.

To mitigate the risks involved before upgrading any macOS bears the question, Do you have a Rescue Plan In-Place?

This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site