The recent textures, it seems, are not saved in preferences (like recent colours).

I know. Colors are just a number, so it's easy to save recent colors. Textures are images with other information that may come from the default library, from .sh3t files or individually imported textures, and this is more complicate to save/read when recent textures list is saved in preferences. I keep it in mind for later.

How about: "The information added that is specific to the newer version will not work with the version of Sweet Home 3D you use now and will be lost if file is saved."

Thanks for the suggestion. I think "will not work" is too severe and let the user think it won't work, whereas it could work partly. But I agree with the warning that the information could be lost if the home is saved.

With 4.4b, it is possible to change the angle of the texture, but not the size/scale.

Yes. The effect of changing the texture size may depend on whether the 3D model defines some texture coordinates or not. I think this concept is complicate to explain to beginners and even to advanced users because a 3D model isn't obliged to define all its texture coordinates if they exist. To avoid some comments like "I changed the texture size but this had no effect on the 3D model", I prefer not to propose this feature at the moment.
There's a solution that would avoid the user to worry about texture coordinates, but it's buggy and I'm not sure it can be exported at OBJ format. Don't worry I keep it in mind and will investigate further if I can circumvent the issue I discovered.

Maybe I missed something in the PluginTexturesCatalog.properties to make textures scalable afterwards?

No, but I hope you understood why from my previous comment.