rpc: add placeholder annotation for deprecated params

diff --git a/src/rpc/mining.cpp b/src/rpc/mining.cpp
index 1d72c699..4465f3a1 100644
--- a/src/rpc/mining.cpp
+++ b/src/rpc/mining.cpp
@@ -536,7 +536,8 @@ static RPCMethod prioritisetransaction()
                 {
                     {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id."},
                     {"dummy", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "API-Compatibility for previous API. Must be zero or null.\n"
-            "                  DEPRECATED. For forward compatibility use named arguments and omit this parameter."},
+            "                  DEPRECATED. For forward compatibility use named arguments and omit this parameter.",
+                        RPCArgOptions{.placeholder = true}},
                     {"fee_delta", RPCArg::Type::NUM, RPCArg::Optional::NO, "The fee value (in satoshis) to add (or subtract, if negative).\n"
             "                  Note, that this value is not a fee rate. It is a value to modify absolute fee of the TX.\n"
             "                  The fee is not actually paid, only the algorithm for selecting transactions into a block\n"
@@ -1092,7 +1093,8 @@ static RPCMethod submitblock()
         "See https://en.bitcoin.it/wiki/BIP_0022 for full specification.\n",
         {
             {"hexdata", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "the hex-encoded block data to submit"},
-            {"dummy", RPCArg::Type::STR, RPCArg::DefaultHint{"ignored"}, "dummy value, for compatibility with BIP22. This value is ignored."},
+            {"dummy", RPCArg::Type::STR, RPCArg::DefaultHint{"ignored"}, "dummy value, for compatibility with BIP22. This value is ignored.",
+                RPCArgOptions{.placeholder = true}},
         },
         {
             RPCResult{"If the block was accepted", RPCResult::Type::NONE, "", ""},
diff --git a/src/rpc/util.h b/src/rpc/util.h
index 82ebea61..70dfd198 100644
--- a/src/rpc/util.h
+++ b/src/rpc/util.h
@@ -170,6 +170,7 @@ struct RPCArgOptions {
     bool skip_type_check{false};
     std::string oneline_description{};   //!< Should be empty unless it is supposed to override the auto-generated summary line
     std::vector<std::string> type_str{}; //!< Should be empty unless it is supposed to override the auto-generated type strings. Vector length is either 0 or 2, m_opts.type_str.at(0) will override the type of the value in a key-value pair, m_opts.type_str.at(1) will override the type in the argument description.
+    bool placeholder{false};             //!< If set, the argument is retained only for compatibility and should generally be omitted.
     bool hidden{false};                  //!< For testing only
     bool also_positional{false};         //!< If set allows a named-parameter field in an OBJ_NAMED_PARAM options object
                                          //!< to have the same name as a top-level parameter. By default the RPC
diff --git a/src/wallet/rpc/coins.cpp b/src/wallet/rpc/coins.cpp
index ab869b0d..e38cb178 100644
--- a/src/wallet/rpc/coins.cpp
+++ b/src/wallet/rpc/coins.cpp
@@ -169,7 +169,8 @@ RPCMethod getbalance()
                 "The available balance is what the wallet considers currently spendable, and is\n"
                 "thus affected by options which limit spendability such as -spendzeroconfchange.\n",
                 {
-                    {"dummy", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "Remains for backward compatibility. Must be excluded or set to \"*\"."},
+                    {"dummy", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "Remains for backward compatibility. Must be excluded or set to \"*\".",
+                        RPCArgOptions{.placeholder = true}},
                     {"minconf", RPCArg::Type::NUM, RPCArg::Default{0}, "Only include transactions confirmed at least this many times."},
                     {"include_watchonly", RPCArg::Type::BOOL, RPCArg::Default{false}, "No longer used"},
                     {"avoid_reuse", RPCArg::Type::BOOL, RPCArg::Default{true}, "(only available if avoid_reuse wallet flag is set) Do not include balance in dirty outputs; addresses are considered dirty if they have previously been used in a transaction."},
diff --git a/src/wallet/rpc/spend.cpp b/src/wallet/rpc/spend.cpp
index 8bb14c47..5686a703 100644
--- a/src/wallet/rpc/spend.cpp
+++ b/src/wallet/rpc/spend.cpp
@@ -342,13 +342,15 @@ RPCMethod sendmany()
                     {"dummy", RPCArg::Type::STR, RPCArg::Default{"\"\""}, "Must be set to \"\" for backwards compatibility.",
                      RPCArgOptions{
                          .oneline_description = "\"\"",
+                         .placeholder = true,
                      }},
                     {"amounts", RPCArg::Type::OBJ_USER_KEYS, RPCArg::Optional::NO, "The addresses and amounts",
                         {
                             {"address", RPCArg::Type::AMOUNT, RPCArg::Optional::NO, "The bitcoin address is the key, the numeric amount (can be string) in " + CURRENCY_UNIT + " is the value"},
                         },
                     },
-                    {"minconf", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "Ignored dummy value"},
+                    {"minconf", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "Ignored dummy value",
+                        RPCArgOptions{.placeholder = true}},
                     {"comment", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "A comment"},
                     {"subtractfeefrom", RPCArg::Type::ARR, RPCArg::Optional::OMITTED, "The addresses.\n"
                                        "The fee will be equally deducted from the amount of each selected address.\n"