Like in the old days can you still fix chains just using a chain tool without split links

yes, but the chain links are a better fix.

Not really.
Anything 9 or higher will almost certainly be using mushroomed heads on the pins. Pretty sure a few 8 did as well.

So once you push the pin out, even without removing it completely, the outer plate you’ve pushed it through will now have a baggy hole, the lip on the pin will also either have been broken off, or crushed.

So the chain will go back together, but you have a hugely weak link in it.
It might work on something like a single speed where there is no side load. But once you get into gears, any badly timed shifts or gear mashing will give you a significant risk of snapping the chain and smashing your nuts. Or even worse, peeling one of the side plates partially off, and smashing your nuts AND destroying your rear mech.

That´s why shimano have oversize joining pins. And everyone else has joining links.

Aye, I’d only use a chain tool on a Sram chain as a get home measure, then fire on a quick link.

Quick links are so readily available and light I carry 2-3 with my trail tool for this reason.

Ok thanks. I have just ordered a bigger cassette and need to lengthen my chain which has only done about 50 clean miles
So quick links it is then .