Well, you can work on it with the existing functionality, with the trust that everything that's hosted on Penpot is first released to the current version on Github. Eventually they will diverge though, and the source code will be a version behind what's on their site. At first the files will be backwards compatible, but soon there will be a must-have feature on the hosted site that will make files incompatible. Then the VC funding will dry up, the hosted site will become paid, and those features and backwards compatibility won't make it to the open sourced code since there won't be the resources to work on the community version. Now you will be stuck, your designs will require the paid software.