This is not an exhaustive list. This is mostly from looking at PRs or IRC meetings. Any mistakes are my own.
- [MERGED] 0.19: Add txids with non-standard inputs to reject filter #19681
- [MERGED] 0.20: Add txids with non-standard inputs to reject filter #19680
- 0.20: Backport wtxid relay #19606
- bitcoin/bitcoin#19569 (comment) (to be opened by jnewbery) with #19610
- [MERGED] Transaction overhaul (#19184 - sipa) (Review by sdaftuar, ajtowns, ariard)
- Erlay (#18261 - naumenkogs) (dependent on #19184) (review by ariard)
- [MERGED] Moving header verification from net_processing to net (#19107 - troygiorshev)
- [MERGED] TORv3 / AddrV2 - BIP155 (#18884, #19628, #19031) (vasild) (review from sipa, troygiorshev, jonatack)
- BIP324 (#18242 - jonasschnelli) (review from jonatack, sipa)
- Allow wtxid-acceptance to the mempool (#19645 - ariard) (needs conceptual review)
- [MERGED] Anchor connections (#17428 - hebasto) (needs conceptual review)
- [MERGED] Unify Send and Receive protocol versions - (#17785 - hebasto)
- Cleanup logic around connection types and enabling outbound & block-relay-only P2P connections in the functional tests (#19315,
#19316- amitiuttarwar) - Shadow simulator v2 from Tor project (https://trac.torproject.org/projects/tor/wiki/org/sponsors/Sponsor38 - dongcarl)
- [MERGED] Populating the peer struct (#19607 - jnewbery) (review from dongcarl)
- Various DNS related things (#18421 - naumenkogs), (#17602 - dongcarl)
- [MERGED] Protect localhost and block-relay-only peers from eviction (#19670 - sdaftuar)
- [MERGED] Serve BIP157 compact filters (#18876 - jnewbery)
- Move remaining application layer data to net processing (#19398,
#19607- jnewbery)
- Package relay (pre-mempool/Memswamp, rbf pinning) - sdaftuar
- Improvements to syncing our tips with more peers (possibly including tx-relay-peer rotation) - sdaftuar
- Alternative transports (#18988 - blocked by multiprocess) - ariard
- Dynamically throttle min-fee-bump for RBF depending on network activity - ajtowns
- Tx rebroadcast privacy - amitiuttarwar
- ASN based banning (#19129 - naumenkogs blocked)
- Splitting into separate blocks and tx p2p networks
- Tx-relay-peer rotation - ariard, sdaftuar
- Increasing the number of block-only relays (which is probably gated on negotiating block-relay connections at connect-time)
- Compact blocks improvements - (maximize likelihood of reconstruction w/ extra pool or prefilled txs, parallel fetching, parallel processing of network messages)
- Not immediately disconnect inbound peers for a block or header that is invalid