Relay has a considerably lower drive impedance than a MOSFET for switching so you end up having to drive the relay with something anyway because the 555 will probably die. Might as well skip the middle man and use a MOSFET which has a crazy high source impedance.

(you need to stick a transistor between the 555 and the relay anyway so you might as well just use a MOSFET)

Nope. 555 has drive capability of over 100mA. More than enough to drive a small relay directly. Just make sure you have a spike suppression diode in circuit.

Take a look at the output switching on different variants of 555s (there are about 6). This isn't always the case. Not only that if you pull lots of current through them, and 100mA is a lot, then the Pd of the sink transistor is pushed a bit hard. I've let the smoke out of a few over the years working on that assumption.

Good tip: measure voltage between the output pin and gnd or Vcc in both states and then have a think.

Plus when you're not an idiot like I was, a MOSFET is cheaper.

Fair point.

I sometimes overestimate the ability of the average electronics hobbyist to properly understand a datasheet.

I don't think many people read the data sheets to be honest even some of the professional engineers. It's mainly gluing canned circuits together and see if they work or not. This is a little difficult if you didn't start with analogue and work up.