Drop subnets #261
Description
They mostly solve the problem of having controlled node groups, but they at the same time exist slightly out of regular placement rules which makes them inconvenient to use while complicating the protocol at the same time. The problem of controlled groups can easily be solved with nspcc-dev/neofs-node#2280 (nspcc-dev/neofs-node#2279 may also be useful in some scenarios), which will fit into the regular placement policy nicely.
This mechanism is not used in the wild, no one really depends on it. We can make our protocol less complex if we're to drop it. The problem mostly is that we've had it in the protocol for a while, so it can't be just removed, it needs to be deprecated, but still kept in many protocol fields to stay compatible (marked as unused/deprecated/always zero/whatever).