Can a version that supports SDK1, such as R420, be permanently retained?

I checked the historical available versions and found that only up to R296 is retained. I don't oppose your upgrade to SDK2, but can you ultimately keep a version that still supports SDK1? I don't need new features, but I simply cannot update all projects using SDK1 plugins one by one.

Therefore, could I request that you retain a version that supports SDK1 for the sake of maintaining old projects? For example, the author of "ProUI" has stopped maintaining it, and while I can modify the plugin myself to ensure my old projects run properly, it is nearly impossible to remove ProUI from all old projects and replace it with built-in components to achieve the same functionality.

you can always open an older version of construct like this:

editor.construct.net/r296

Just make sure that version doesn't collide with your installed plugins, in case they are more recent. Or open an incognito tab, and install proui to work temporarily there.

What I mean is that I can still open version R296 now, but what will happen after July when SDK1 is completely unsupported? Old projects may not be accessible at all. What I want to know is whether a version that still supports SDK1 can be retained after full SDK1 deprecation, so that we can maintain and make minor changes to our old projects without needing new features.

What I mean is that I can still open version R296 now, but what will happen after July when SDK1 is completely unsupported? Old projects may not be accessible at all. What I want to know is whether a version that still supports SDK1 can be retained after full SDK1 deprecation, so that we can maintain and make minor changes to our old projects without needing new features.

SDK v2 is available only r388.2, which was the last stable sdk v1 in my opinion. As long as you remain r388.2 or less, you should be good to open your old projects. After that release, scirra made multiple changes that also affected the sdk v1 and I'd rather avoid to keep it "clean".

As chadory said, this seems to be supported indefinitely. On the other hand, the LTS version will be available for around 2 years, it will support sdk v1 and sdkv2 and will contain the latest c3 improvements.