Spring 20 LWC Cannot add a property x, object not extensible
Yes, this has been asked over and over here, and likely elsewhere as well. However, I'm not marking your question as a duplicate, as a matter of a slightly different nature in the wording deserves an answer.
The "cacheable" attribute that allows the same method with the same parameters to not have to round-trip to the server repeatedly (thus improving performance on idempotent method calls) requires that the object in the cache either be copied or marked read-only, because otherwise a developer could corrupt the cache.
The read-only option provides much better performance for large responses, so this was the design chosen. If you need a modifiable object, you are indeed expected to copy it to a new object, recursively if necessary.
You're right, as far as I can tell, it's a lacuna in the documentation (I think I may have wanted an excuse to use lacuna, I love that word). At this point, it's pretty well known for people who have asked about it, but someone should probably ask @salesforcedocs over on Twitter about it.
You'll want to make the appropriate changes to your code before Spring 20 goes live in production. Either the JSON.stringify/parse or rest-copy (depending on deep or shallow copy) should be forward compatible with all future releases of LWC.
My team has faced the issue as you with multiple LWC components on Spring '20 preview org.
We found a post from Salesforce employee on Salesforce Partners forum which states that the problem has been acknowledged and the fix should be deployed this week.
The change is made by design and is about a unifying response from LDS, which in other areas (@wire) is immutable. Although the update broke many components, so Salesforce team decided to revert it. However, as I understand - it can come back in one of the next few releases, so we should make components future proof with object cloning.