https://gol.hatsya.co.uk/census/b3s23/s ... ffset=1000

Some of the unsynthesised ones appear easy, and are probably due to a component that Shinjuku missed (it only bothers with components that are on the optimal path of some still-life). It should suffice to solve a few of the easy unsynthesised ones (e.g. by using a tub-to-boat converter), and rerun the 'transfer learning' script to fill in many of the other gaps.

EDIT: CatIsFluffy provided a good starting point, which (when added to Shinjuku) should knock off many of the remaining unsynthesisables:

`x = 13, y = 18, rule = B3/S23`

4b2o$5bo$3bo$b4o$o$obo8bo$bobo6bo$2bobo5b3o$3bo7$9b2o$9bobo$9bo!