React Native iOS Accessibility Font Scaling Workaround

RCTAccessibilityManager+FontMultipliers.m

//
//  RCTAccessibilityManager+FontMultipliers.m
//
// This category is a workaround React Native's default Font Sizing Accessibility settings
//
// Swizzle the getter of the font size multipliers so we can limit the max size.
//

#import "RCTAccessibilityManager+FontMultipliers.h"
#import <objc/runtime.h>

@implementation RCTAccessibilityManager (FontMultipliers)

+ (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    Class class = [self class];
    
    SEL originalSelector = @selector(multipliers);
    SEL swizzledSelector = @selector(custom_accessibility_font_multipliers);
    
    Method originalMethod = class_getInstanceMethod(class, originalSelector);
    Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
    
    // When swizzling a class method, use the following:
    // Class class = object_getClass((id)self);
    // ...
    // Method originalMethod = class_getClassMethod(class, originalSelector);
    // Method swizzledMethod = class_getClassMethod(class, swizzledSelector);
    
    BOOL didAddMethod =
    class_addMethod(class,
                    originalSelector,
                    method_getImplementation(swizzledMethod),
                    method_getTypeEncoding(swizzledMethod));
    
    if (didAddMethod) {
      class_replaceMethod(class,
                          swizzledSelector,
                          method_getImplementation(originalMethod),
                          method_getTypeEncoding(originalMethod));
    } else {
      method_exchangeImplementations(originalMethod, swizzledMethod);
    }
  });
}

#pragma mark - Method Swizzling

- (NSDictionary<NSString *, NSNumber *> *)custom_accessibility_font_multipliers {
  return @{UIContentSizeCategoryExtraSmall: @0.823,
                     UIContentSizeCategorySmall: @0.882,
                     UIContentSizeCategoryMedium: @0.941,
                     UIContentSizeCategoryLarge: @1.0,
                     UIContentSizeCategoryExtraLarge: @1.118,
                     UIContentSizeCategoryExtraExtraLarge: @1.235,
                     UIContentSizeCategoryExtraExtraExtraLarge: @1.353,
                     UIContentSizeCategoryAccessibilityMedium: @1.5, // cap at 1.5x
                     UIContentSizeCategoryAccessibilityLarge: @1.5,
                     UIContentSizeCategoryAccessibilityExtraLarge: @1.5,
                     UIContentSizeCategoryAccessibilityExtraExtraLarge: @1.5,
                     UIContentSizeCategoryAccessibilityExtraExtraExtraLarge: @1.5};
}

@end

You missed the Header file:

#import "React/RCTAccessibilityManager.h"

@interface RCTAccessibilityManager (FontMultipliers)

@end