I recently stumbled on a blind SSTI injection on a bug bounty program (no output nor stack trace, only 500 status code on invalid syntax)
The version was up to date and it was not possible to RCE because the conf was following best practices and there is no public sandbox bypass on the latest version. So was it possible to do stuff anyway ? Yes I found some nice gadgets to enumerate all accessible variables from the engine, read data blindly or perform some DoS.
This is not meant to be complete, you will find classic payloads for freemarker on other cheat sheets this is only the new stuff from my research which is not public anywhere else