staking: deprecate obsolete base reward rate field (#4651)

## Describe your changes

This PR deprecate the
`core.component.staking.v1.StakeParameters.base_reward_rate` field. This
field is vestigial and unused. The actual chain base reward rate is
computed downstream of the distributions component budget issuance for
the epoch.

## Checklist before requesting a review

- [x] If this code contains consensus-breaking changes, I have added the
"consensus-breaking" label. Otherwise, I declare my belief that there
are not consensus-breaking changes, for the following reason:

> Technically could be client breaking but I don't think it is used
anywhere

crates/core/app/src/params/change.rs

@@ -94,7 +94,6 @@ impl AppParameters {
             stake_params:
                 StakeParameters {
                     active_validator_limit,
-                    base_reward_rate: _,
                     slashing_penalty_misbehavior: _,
                     slashing_penalty_downtime: _,
                     signed_blocks_window_len,
@@ -190,7 +189,6 @@ impl AppParameters {
             stake_params:
                 StakeParameters {
                     active_validator_limit,
-                    base_reward_rate,
                     slashing_penalty_misbehavior,
                     slashing_penalty_downtime,
                     signed_blocks_window_len,
@@ -223,10 +221,6 @@ impl AppParameters {
                 *active_validator_limit > 3,
                 "active validator limit must be at least 4",
             ),
-            (
-                *base_reward_rate >= 1,
-                "base reward rate must be at least 1 basis point",
-            ),
             (
                 *slashing_penalty_misbehavior >= 1,
                 "slashing penalty (misbehavior) must be at least 1 basis point",

crates/core/component/stake/src/params.rs

@@ -10,8 +10,6 @@ pub struct StakeParameters {
     pub unbonding_delay: u64,
     /// The number of validators allowed in the consensus set (Active state).
     pub active_validator_limit: u64,
-    /// The base reward rate, expressed in basis points of basis points
-    pub base_reward_rate: u64,
     /// The penalty for slashing due to misbehavior, expressed in basis points squared (10^-8)
     pub slashing_penalty_misbehavior: u64,
     /// The penalty for slashing due to downtime, expressed in basis points squared (10^-8)
@@ -36,7 +34,6 @@ impl TryFrom<pb::StakeParameters> for StakeParameters {
             active_validator_limit: msg.active_validator_limit,
             slashing_penalty_downtime: msg.slashing_penalty_downtime,
             slashing_penalty_misbehavior: msg.slashing_penalty_misbehavior,
-            base_reward_rate: msg.base_reward_rate,
             missed_blocks_maximum: msg.missed_blocks_maximum,
             signed_blocks_window_len: msg.signed_blocks_window_len,
             min_validator_stake: msg
@@ -58,7 +55,7 @@ impl From<StakeParameters> for pb::StakeParameters {
             missed_blocks_maximum: params.missed_blocks_maximum,
             slashing_penalty_downtime: params.slashing_penalty_downtime,
             slashing_penalty_misbehavior: params.slashing_penalty_misbehavior,
-            base_reward_rate: params.base_reward_rate,
+            base_reward_rate: 0,
             min_validator_stake: Some(params.min_validator_stake.into()),
             unbonding_delay: params.unbonding_delay,
         }
@@ -78,8 +75,6 @@ impl Default for StakeParameters {
             slashing_penalty_misbehavior: 1000_0000,
             // 1 basis point = 0.01%
             slashing_penalty_downtime: 1_0000,
-            // 3bps -> 11% return over 365 epochs
-            base_reward_rate: 3_0000,
             // 1 penumbra
             min_validator_stake: 1_000_000u128.into(),
         }

crates/proto/src/gen/penumbra.core.component.stake.v1.rs

@@ -769,6 +769,7 @@ pub struct StakeParameters {
     #[prost(uint64, tag = "2")]
     pub active_validator_limit: u64,
     /// The base reward rate, expressed in basis points of basis points
+    #[deprecated]
     #[prost(uint64, tag = "3")]
     pub base_reward_rate: u64,
     /// The penalty for slashing due to misbehavior.

proto/penumbra/penumbra/core/component/stake/v1/stake.proto

@@ -310,7 +310,7 @@ message StakeParameters {
   // The maximum number of validators in the consensus set.
   uint64 active_validator_limit = 2;
   // The base reward rate, expressed in basis points of basis points
-  uint64 base_reward_rate = 3;
+  uint64 base_reward_rate = 3 [deprecated = true];
   // The penalty for slashing due to misbehavior.
   uint64 slashing_penalty_misbehavior = 4;
   // The penalty for slashing due to downtime.
@@ -408,4 +408,4 @@ message EventSlashingPenaltyApplied {
   uint64 epoch_index = 2;
   // The penalty amount after slashing.
   Penalty new_penalty = 3;
-}
+}