Both their
low and
high feerate estimatimates as well as what properties their online wallet transactions could and certainly don't have is known. For example:
- Only spending P2PKH outputs (thus not spending SegWit nor multisig)
- Not signaling explicit RBF
- Not setting a locktime
- BIP-69 compliant (IIRC parts of their wallet are on GitHub, don't have the link handy right now)
- Not more than two outputs are created (one payment and maybe one change output)
Maybe also "full length signatures" -- not doing the nonce grinding to reduce the signature size the bitcoin core does? Though that won't filter much except on txn that have many inputs and BIP-69 will probably already eliminate bitcoin core for those.
If there are two outputs, at least one of them will be P2PKH.