substrate-shim.c

#include <objc/runtime.h>

extern void *SubGetImageByName(const char *filename) __asm__("SubGetImageByName");;
void *MSGetImageByName(const char *filename) {
    return SubGetImageByName(filename);
}

extern void *SubFindSymbol(void *image, const char *name) __asm__("SubFindSymbol");
void *MSFindSymbol(void *image, const char *name) {
	return SubFindSymbol(image, name);
}

extern void SubHookFunction(void *symbol, void *replace, void **result) __asm__("SubHookFunction");
void MSHookFunction(void *symbol, void *replace, void **result) {
	SubHookFunction(symbol, replace, result);
}

extern void SubHookMessageEx(Class _class, SEL sel, IMP imp, IMP *result) __asm__("SubHookMessageEx");
void MSHookMessageEx(Class _class, SEL sel, IMP imp, IMP *result) {
	SubHookMessageEx(_class, sel, imp, result);
}

to build shim:

1. unarchive substitute.0.dylib from tweaksupport.tar, save as substitute.dylib
2. Get this file
3. xcrun -sdk iphoneos cc -dynamiclib -arch arm64 substrate-shim.c -L$PWD -lsubstitute -install_name /Library/Frameworks/CydiaSubstrate.framework/CydiaSubstrate -o CydiaSubstrate
4. jtool --sign --inplace CydiaSubstrate or ldid or ldid2 or whatever
5. on device: mkdir -p /Library/Frameworks/CydiaSubstrate.framework
6. copy CydiaSubstrate you've built to /Library/Frameworks/CydiaSubstrate.framework
7. Tweaks depending on CydiaSubstrate would likely work -- when you move their files from /Library/MobileSubstrate/DynamicLibraries to /bootstrap/Library/SBInject ofc (no need to mess with install_name_tool though)
   You can just make a symlink from /Library/MobileSubstrate/DynamicLibraries to /bootstrap/Library/SBInject.