Do you think bike frame size is less important these days?

true - but do the research well then the lbs should be looking at you and giving the stem or seatpost a tweak or two, nothing more..

I'm contextualising this in terms of buying an old bike or frame of ebay or on here btw, seeing as that's what so many people are plainly doing and it's what I do as I don't like the OTPs and can't afford me a nice Chas Roberts etc

you obviously know this already. the key is to find out your effective top tube plus stem length. you do this by either getting fitted or trial and error. i know i need a 55-55.5 plus 120 stem for track 55.5-56.5 plus 120 for road. this gives me leeway for sensible stem length if i stick to 55.5-56.5cm effective top tubes.
it's not difficult to work this shit out.

once you know this you can tell instantly if another bike is going to fit. give or take slight changes in seat angle.

another pet hat is 'nippy in town' just as much rapid changing of direction goes on with track and road riding using 130mm stems. boonen uses a 140, and how long is the stem on theo bos's bike?