According to its Technical Specifications, your iPhone supports
- MagSafe wireless charging up to 25W with 30W adapter or higher
- Fast-charge capable: Up to 50% charge in around 30 minutes with 20W adapter or higher paired with USB-C charging cable, or 30W adapter or higher paired with MagSafe Charger (all available separately)
So a 30W adapter would let you fast-charge either with USB-C or with MagSafe. But if someone told you that a 20W adapter would cause your battery to die quickly, they were probably feeding you a load of bull.