After a long wait we have published new testing builds of Synfig Studio (1.5.3) - Release Release build (v1.5.3) · synfig/synfig · GitHub
It contains all latest contributions to Synfig code.
We are moving towards the stable release, so your help on testing those builds is much appreciated! Please report all critical regressions to Issues · synfig/synfig · GitHub
Just bumping the topic to check if you, dear Synfig users 
, are testing this release 
Repeating: please report any issues, specially regressions (stuff that works on 1.0.x, 1.2.x and -mainly- 1.4.x versions, but not anymore on 1.5.3)
I did some surface-level tests and didn’t find any regressions. I am currently using my own Synfig build based on commit 768188305412e231309cddf467c58bcc304863d9 and I did find some long-standing bugs I am going to report when I finish my Spy vs Spy cartoon (probably in late autumn).