In-house work is still an asset of the respective company, as long as they paid their workers.
Contract work is all copyright by their respective authors after the contract expired - and will remain this way after 75 years after their death or something of similar timescale.
Most publishing deals expired after a given time period like 2-5 years after first publication, or if if no publication happened within a given time period.
So buying games produced in-house by say Ocean are safe bets from Atari (Infogrames bought Ocean, Inforgrames bought the brand Atari from Hasbro and trade under the Atari name...), while buying my games or some other guys from this board would result in a lawsuit
Pretty simple, but each case has to be checked individually, most companies published both contract and in-house works.