My library contains code like:

NettyChannelBuilder ncb = NettyChannelBuilder.forTarget(...);
ncb.setMaxInboundMessageSize(...);
ncb.eventLoopGroup(...);
// ...
ManagedChannel channel = ncb.build();

which breaks when moving from pre-1.33.0 to 1.33.0, unless recompiled (and then the new binary would not be compatible with versions < 1.33.0). This is because a class (AbstractManagedChannelImplBuilder) was removed from the direct superclass heriarchy:

Changing the direct superclass or the set of direct superinterfaces of a class type will not break compatibility with pre-existing binaries, provided that the total set of superclasses or superinterfaces, respectively, of the class type loses no members.

I know that technically netty transport is still considered experimental (per #1784), but (a) it's the primary transport and (b) it's been that way for more than 4 years.

Was this intentional/known or is it a case of "it's experimental, tough!"?

It should be easy to avoid I think by for example just adding an empty abstract class AbstractManagedChannelImplBuilder extends ForwardingChannelBuilder... maybe in a 1.33.1? :)