Check for iOS 26 (can be modified for other versions too) and conditional glass effect to manage backward compatibility.

AppConfig.swift

struct AppConfig {
    static var isCompilerAtLeast62: Bool {
#if compiler(>=6.2)
        return true
#else
        return false
#endif
    }

    // This will only return true when built from Xcode 26 and for iOS 26 on other version it will return false.
    static var isOS26Available: Bool {
#if os(iOS)
        if #available(iOS 26.0, *) {
            return isCompilerAtLeast62
        } else {
            return false
        }
#elseif os(watchOS)
        if #available(watchOS 26.0, *) {
            return isCompilerAtLeast62
        } else {
            return false
        }
#endif
    }
}

View+Extension.swift

// MARK: - Glass effect
extension View {
    @ViewBuilder
    func glassEffect<S: Shape, Content: View>(
        in shape: S? = nil as Capsule?,
        color: Color? = nil,
        isInteractive: Bool = false,
        fallback transform: (Self) -> Content
    ) -> some View {
#if compiler(>=6.2)
        if #available(iOS 26.0, *) {
            applyGlassEffect(shape: shape, color: color, isInteractive: isInteractive)
        } else {
            applyFallback(shape: shape, transform: transform)
        }
#else
        applyFallback(shape: shape, transform: transform)
#endif
    }

    @ViewBuilder
    func glassEffect<S: Shape>(
        in shape: S? = nil as Circle?,
        color: Color? = nil,
        isInteractive: Bool = false
    ) -> some View {
#if compiler(>=6.2)
        if #available(iOS 26.0, *) {
            applyGlassEffect(shape: shape, color: color, isInteractive: isInteractive)
        } else {
            applyFallback(shape: shape, transform: { $0 })
        }
#else
        applyFallback(shape: shape, transform: { $0 })
#endif
    }

#if compiler(>=6.2)
    @available(iOS 26.0, *)
    @ViewBuilder private func applyGlassEffect<S: Shape>(
        shape: S?,
        color: Color?,
        isInteractive: Bool
    ) -> some View {
        let baseStyle = isInteractive ? Glass.regular.interactive() : Glass.regular
        let glassStyle = color.map { baseStyle.tint($0) } ?? baseStyle
        if let shape {
            self.glassEffect(glassStyle, in: shape)
        } else {
            self.glassEffect(glassStyle)
        }
    }
#endif

    @ViewBuilder
    private func applyFallback<S: Shape, Content: View>(
        shape: S?,
        transform: (Self) -> Content
    ) -> some View {
        if let shape {
            transform(self).clipShape(shape)
        } else {
            transform(self)
        }
    }
}