Hi Will,

I shared some numbers and feedback as comment [0] on your PR wanted to
answer question 1. for completeness here too.

> Its original intention was to be publicly callable, but it is now (in
Bitcoin Core) gated behind stricter Net Permissions which make it
accessible to trusted peers only.

Bitcoin Core does only gate processing of mempool messages on
NetPermissionFlags::Mempool when bloom filters are disabled [1]. While
these are disabled by default, more than 20% (see PR comment) of nodes
on the network have bloom filters enabled. These nodes all respond to
mempool messages with INV messages.

> 1. Are there any parties who still directly rely on the BIP35 P2P
`mempool` message?

I've been receiving on average about 20 mempool messages per hour to a
well-connected NODE_BLOOM Bitcoin Core node. I've seen multiple messages
from the user agent /BitcoinKit:0.1.0/, /bitcoinj:0.*.*/Bitcoin
Wallet:*/, /WalletKit:0.1.0/, and /bread:2.1/. Similarly, the node
responds to the clients with INVs up to the max number of 50k entries
and with smaller (bloom) filtered INVs.


0xB10C

[0]: https://github.com/bitcoin/bitcoin/pull/27426#issuecomment-1529678174
[1]:
https://github.com/bitcoin/bitcoin/blob/d89aca1bdbe52406f000e3fa8dda12c46dca9bdd/src/net_processing.cpp#LL4603C52-L4603