[nomaster] SI-7195 minor version mustn't introduce warnings

We want 2.10.1 to be a drop-in replacement for 2.10.0,
so we can't start warning where we weren't warning in 2.10.0.

See SI-5954 (#1882, #2079) for when it was an implementation restriction,
which was then weakened to a warning. It's now hidden behind -Ydebug.

review by @jamesiry