If I run an app with a Message Filter Extension on a handset with iOS 18.2 then it runs fine, however if I run the exact same app with no changes on a different phone which has iOS 17.6.1 installed then the following error occurs when the extension is enabled within Settings:
dyld[631]: Symbol not found: _$sSo40ILMessageFilterCapabilitiesQueryResponseC14IdentityLookupE21promotionalSubActionsSaySo0abI6ActionVGvs