I opened an old C3 project to read an internal XML file. I was surprised to find the message saying that Construct 2 runtime will be deprecated in July 2021. I understand the need for this. I still have about 5 months to fix what I need, but I was unaware of this until now. I mostly have to remove a few plugins and switch to C3 runtime, it is simple to do. I learned a lot since I first made it so I would need to restructure the project anyways when I am trying to finish it. I put years into the old project and it would have hurt a lot to of lost it. The warning says future versions of C3 may not be able to open the project. I am not sure if I would have lost the project completely or just parts of it would have been removed.
I am just letting you all know in case you didn't know about it. Something like this probably had an announcement or mentioned somewhere, but I was unaware of it. I did a quick forum search for legacy and didn't see anything in the titles, I probably could have tried harder though.
Be safe and open old C3 projects in the newest version of C3.