Last time it was the Python compiler that got stricter, in the latest update it's Fusion 360 that got stricter on API calls.
Previously, you could get away with trying to set a read-only property - it had no effect but did not cause an error either.
This change is good because now you'll get an error and will be notified as you're developing your add-in when you're trying to do something that does not make sense.
The only problem is if you happen to have a published add-in that includes such a call 🙄
If your add-in stopped working (like mine 🤫 ) in the latest Fusion 360 update then this might be it.