As you’ve already discovered, plant tracker is no longer being supported. I’m not aware of any alternative national platforms/apps at the moment, but it is possible to create your own local database and data collection form. The new Outfall Safari Guide & Resource Pack is a great example of setting up a local data collection platform.
We recommend Survey123 for ArcGIS. This mobile data collection tool is free with an ArcGIS online subscription and relatively simple to set up, with lots of online tutorials available. It will allow you to submit records via desktop or mobile app and can be used offline when working in the field.
If you don’t have an ArcGIS online subscription, there are some other free or low cost alternatives, such as epicollect or SurveyCTO.
Hope that is useful. Please get in touch if you have any more questions,