I have finished the 2015–2025 papers for Polity and Economy. Is it worth going back to the 1990s papers, or should I focus on current affairs?

I wouldn't go back to the 1990s as a default. The pattern and even some of the syllabus emphasis has shifted. I use older papers only when I'm practising a topic (say, polity) and want more questions, not as a full year-by-year project. 2011 or 2013 onwards is already a lot if you actually review them.

agreed. I tried 1990s polity papers for a week and it ate time I should have spent on CA and environment. maybe a few questions if you're done with the last 10–12 years, not before.

For economy I found the last 8–10 years more useful than the very old ones, because the policy context is closer. I'll still open an older question if it is on a static concept (like a basic monetary tool), but I wouldn't plan a full 1995–2010 pass before 2027.

I'm in the same place as the original post – 2015–2025 done for two subjects. I'll do the remaining subjects for that window first, and only then dip into older papers for weak topics. Thanks for asking this, I was about to start 1998 out of anxiety.