How could the Bitcoin protocol be modified so that pruned nodes could still participate in BIP 37 SPV? According to Jameson Lopp (https://www.coindesk.com/spv-support-billion-bitcoin-users-sizing-scaling-claim/), allowing pruned nodes to advertise that they support BIP 37 would be impossible without "extensive protocol changes." What changes would make this possible?
Pruned peers do not have the full blockchain history and therefore have only limited value for SPV clients.
With Bitcoin Core 0.16, pruned peers will announce their service via NODE_NETWORK_LIMITED (see BIP159). BIP159 is not a consensus change.
This will allow SPV clients to connect to pruned peers. But, since they only signal for the last 288 blocks, SPV/BIP37 clients interested to load history older than 1 day of blocks would still require to connect to traditional peers.
In the future, pruned nodes may have a way to signal how many blocks they will keep on disk (without adding a fingerprinting vulnerability).
Note that BIP37 is a controversial concept. It has significant privacy downsides (see Jonas Nick's research).
A far better concept is &quot;Client side filtering&quot;. See the upcoming BIP157 and BIP158.