I have a vintage pie recipe that calls for acid syrup. Is this still available, and if not, what is a reasonable substitution?

sorry, forgot to link the article: http://www.cooksinfo.com/ascorbic-acid

I wonder if it is to prevent darkening of fruits. This article about culinary and nutritional uses for ascorbic acid mentions rose-hip syrup being used in Britain during WWII as a source of vitamin C (end of paragraph, "cooking tips"). Is that around the era of your recipe?

Check "golden syrup" (found often in British recipes) in Wikipedia or other descriptions and compare it to your recipe. Described as using avid in its production, so maybe it was sometimes named after the process.
If yes, available in some grocery & specialty stores in USA & Canada.