Is it safe to ride a steel bike with a dented/buckled frame?
All steel frames of any age get a few dings from serious use.

The failure point for steel frames is usually the highest stress points: the joins. That is why we used to have fancy tubing sets likes Reynolds 351 double butted, and I notice that your frame does too.

So, if the dent is away from the joins I say the frame is ok for general use. Steel is quite a forgiving material, but I wouldn't be doing bunny hops on it :-)

If the dent is within 25% of the tube length, but more than 10cm (4 in) from a join, then I would be using it, but cautiously. If it's closer than 10cm from a join then I would get a shop to look at it.

I would add that I've ridden high-end bikes for decades after their first (heart breaking) dents. One of them has just died, but a fork failed (at the join), not the dented frame.