constexpr float flFrequency = 1.f / 0.5f;
float flStep = flFrequency * csgo.m_globals->m_frametime;
float flOpacity{};
int iAlpha;
// Here you can fade after x amount of time.
flTracerTime > x seconds ? flOpacity -= flStep : flOpacity += flStep;
// Clamp...
math::clamp( flOpacity, 0.f, 1.f );
// Use this in yor tracer colour.
iAlpha = 0xff * flOpacity;
// Example:
Draw::Line(x, y, Color(r, g, b, iAlpha));
// If this doesn't work I will make a proper tracer for you tommorow