Based on the Central Motor Vehicles Rules and recent Supreme Court and RTO enforcement patterns. Legal means broadly permitted, Grey area means legal only under specific conditions, Illegal means it will attract a challan or worse.
This isn't legal advice. Enforcement varies by state and even by traffic inspector — some cities run visible crackdowns (Delhi, Bengaluru, Kerala, Hyderabad and Punjab have all seen major drives on structural mods), while smaller towns may be more lenient in practice. When a modification needs RTO approval, get it in writing and have the RC updated — a verbal "it's fine" from a mechanic isn't a legal defence.