As you can see, the blocking for update in the packages is that they failed to compile for i386 and s390x.
So it is not published.
Who will still use a i386 and who would be using Synfig on an IBM Mainframe? Mistery!
But as it is the policy of Debian, it is blocked (either it works everywhere or it is not updated).
So better to use alternatives than wait for as I guess your CPU is not so “exotic”.
Or to compile it yourself.
Note that you could even create your own .deb with CMake script, but this would be limited to your current OS distro (cpu/bitness/os version) as we had the case for the AppImage by @zainal (see here)
