New user documentation

- [Reduce memory](https://github.com/bitcoin/bitcoin/blob/master/doc/reduce-memory.md)

suggests configuration tweaks for running Bitcoin Core on systems with

limited memory. (#16339)

- `getblockfilter` gets the BIP158 filter for the specified block. This

RPC is only enabled if block filters have been created using the

`-blockfilterindex` configuration option. (#14121)

New settings

- `-blockfilterindex` enables the creation of BIP158 block filters for

the entire blockchain. Filters will be created in the background and

currently use about 4 GiB of space. Note: this version of Bitcoin

Core does not serve block filters over the P2P network, although the

local user may obtain block filters using the `getblockfilter` RPC.

(#14121)

Updated settings

- `whitebind` and `whitelist` now accept a list of permissions to

provide peers connecting using the indicated interfaces or IP

addresses. If no permissions are specified with an address or CIDR

network, the implicit default permissions are the same as previous

releases. See the `bitcoind -help` output for these two options for

details about the available permissions. (#16248)

- `getdescriptorinfo` now returns an additional `checksum` field

containing the checksum for the unmodified descriptor provided by the

user (that is, before the descriptor is normalized for the

`descriptor` field). (#15986)

- `walletcreatefundedpsbt` now signals BIP125 Replace-by-Fee if the

`-walletrbf` configuration option is set to true. (#15911)

GUI changes

- Provides bech32 addresses by default. The user may change the address

during invoice generation using a GUI toggle, or the default address

type may be changed by the `-addresstype` configuration option.

(#15711, #16497)

Deprecated or removed configuration options

- `-mempoolreplacement` is removed, although default node behavior

remains the same. This option previously allowed the user to prevent

the node from accepting or relaying BIP125 transaction replacements.

This is different from the remaining configuration option

`-walletrbf`. (#16171)

- `generate` is now removed after being deprecated in Bitcoin Core 0.18.

Use the `generatetoaddress` RPC instead. (#15492)

- `getrpcinfo` now returns a `logpath` field with the path to

`debug.log`. (#15483)

- If a Tor hidden service is being used, Bitcoin Core will be bound to

the standard port 8333 even if a different port is configured for

clearnet connections. This prevents leaking node identity through use

of identical non-default port numbers. (#15651)

Mempool and transaction relay

- Allows one extra single-ancestor transaction per package. Previously,

if a transaction in the mempool had 25 descendants, or it and all of

its descendants were over 101,000 vbytes, any newly-received

transaction that was also a descendant would be ignored. Now, one

extra descendant will be allowed provided it is an immediate

descendant (child) and the child's size is 10,000 vbytes or less.

This makes it possible for two-party contract protocols such as

Lightning Network to give each participant an output they can spend

immediately for Child-Pays-For-Parent (CPFP) fee bumping without

allowing one malicious participant to fill the entire package and thus

prevent the other participant from spending their output. (#15681)

- Transactions with outputs paying v1 to v16 witness versions (future

segwit versions) are now accepted into the mempool, relayed, and

mined. Attempting to spend those outputs remains forbidden by policy

("non-standard"). When this change has been widely deployed, wallets

and services can accept any valid bech32 Bitcoin address without

concern that transactions paying future segwit versions will become

stuck in an unconfirmed state. (#15846)

- Legacy transactions (transactions with no segwit inputs) must now be

sent using the legacy encoding format, enforcing the rule specified in

BIP144. (#14039)