Old variables format doesn't work with newer versions of Ren'Py

[Renmazuo]

Steps taken:

1. Download the latest version of Ren'Py (in my case 6.13.7.1646) and extract the DSE downloadable from renpy.org in the projects folder;
2. Run the DSE.

Results:

As soon as the first variable is displayed in game you will see that it shows like this:

It's day %(day)d.

Would it be possible to make it so that older projects that use the old way of calling variables are still compatible with the newer Ren'Py versions? I found this while experimenting with the DSE to figure it out and explain it to someone else who wants to create their game with it but seems to have difficulties making sense of the documentation. This bug can be fairly confusing for people who don't have any programming knowledge or help and may lead (or may have lead many before) them to just drop the DSE due to the negative first approach.
Additionally, this breaks cookbook recipes (eg: ingame messages, inventory and money system, simple on-screen inventory and probably more). If you change the examples themselves they will break for older versions of Ren'Py that people may still be using and cookbook examples are more likely to be used by novice programmers than people who can make sense of the problem.

Since it's very unlikely someone would type the old method for text interpolation without meaning to, couldn't it just be on by default as well?

[Renmazuo (siebtzen)]

Corrected the day variable and some typos.

[Renmazuo (siebtzen)]

Changed summary as this isn't exclusive to the DSE and description as it breaks code examples that shouldn't be changed as they wouldn't work with older versions of Ren'Py any more.

[Tom Rothamel (renpytom)]

I think this is sound, so I've re-enabled old-style interpolation.