tl;dr: There's nothing to worry about, this was expected and planned for.
The mirclient APIs used in Unity8 and 16.04 based applications is not dropped, and does not require a build-time option to enable it.
It simply needs switching on with a configuration option at runtime. (There are many ways that can be supplied, for example, adding code to QtMir and unity-system-compositor that fakes passing this as a "command-line option" to Mir.)
For a bit of perspective, these APIs have been deprecated for two years, and the UBports developers have been aware of that. Similarly, the Mir developers have been aware that they are used by UBports and have continued to build and test them.
Everyone working on this software stack has always known that UBports needs to move off these APIs at some point, but that cannot happen until there is a viable alternative. And that requires moving to a more recent version of Mir (at least Mir 1.3 for this Mir support for Wayland).
The change to "new Mir" is progressing, and is available on the "edge" channel. Delivering that will allow the migration to Wayland to start.
There have been discussions (here and here) about this migration and, when the time comes, the Mir team has the expertise, willingness and availability to help UBport move from mirclient APIs to Wayland protocols.
There may be a time during this transition where UBports cannot ship with the latest release of Mir, but the way forward is clear and the problems to be addressed are far smaller than ones that have already been overcome.