User blog comment:AnaCordeiro159/About the "migration"/@comment-1038387-20201012101506

To reiterate what has been said already, elsewhere:

Chat is not compatible with the new version. It's a hack of outdated code with limited functionality, and bringing it up to the current is not an option. If your wiki uses it, it sucks. But in the big picture, usage was extremely limited, so that it did not justify a substantial investment. Resources are limited and this was an investment they would get little to no return on; Chat does not pay for itself.

To make it "optional" on UCP would still require the investment, so that's not going to happen.