Isos is right in the meaning of "multirole". During cold war the aircrafts were highly specialized, but not due less computer power...it was because specialzed aircraft are simply better in their job! And old aircraft with up to date modern avionic are still very deadly. Multirole comes into play in times of cost reduction. And this was the only reason for the F-35...it was a political decision, not a solution prefered by US Military.
Old computing chips were slow and not able to process a lot of data very quickly... a modern smart wrist watch has more computing power than was used to get people to the moon, but analog computers were used that were hardwired just to do a specific job. A calculator doesn't need a super CPU just to do basic calculations and even the most ancient computer chips from the 1970s and 1980s can perform calculations faster than a person can push buttons on a calculator.
The point is that the exponential increase in processing power meant they could start to do much more than the old calculator hard wired computers, so features could be added using software instead of adding new bits of hardware.
Multirole fighters only really became possible in the late 1980s and early 1990s with processors like the 286 and 386 and 486 chips... which are pre pentium dinosaurs but the requirements of a fighter plane were not that big.... they had dedicated computers processing radar information, so from the target distance and direction and speed interception points were simple calculations, and the times needed for the target to be illuminated to guide a SARH missile like SPARROW could be calculated too and managed by a simple computer. What started to require serious processing power is digital video of ground targets and transmitting such information back to HQ instead of using wet film for recon that had to be flown back to base and developed before the intel could be used...
Most of the time a multirole aircraft is not as valuable as it sounds... accountants love them because on paper it saves money, but it often also makes them more expensive so you can afford less than you probably want.
Having said that a MiG-21 and a MiG-23 and a MiG-27 and an Su-17 could all be replaced by a MiG-29SMT... it has better weapon options than any of those aircraft, and while its single barrel 30mm cannon doesn't offer the same fire power as the 30mm gatling on the MiG-27 or the twin 30mm guns on the Fitters, and the MiG-29SMT does not have the acceleration of the MiG-27 it has weapons that exceed the performance of any carried by the older generation aircraft... it is a better dogfighter than the MiG-21, it is a better interceptor than the MiG-23 because it can carry much more potent missiles and has a better radar and IRST, and in ground attack it is as good as the MiG-27M or MiG-27K... and would be rather more effective in the swing role in that it could hit targets with TV or laser guided bombs or missiles and then defend itself with R77 and R73 AAMs in the same mission.
New 5th gen and 4++ gen aircraft are now being called omnirole where they are expected to perform a range of roles within one mission using extensive sensors and datalinks to find targets like a recon aircraft... engage enemy aircraft like a fighter, or any ground defences like a SEAD aircraft... monitor or record the results of those attacks and pass the information to HQ so it can be decided if follow up attacks are needed and then return to base for rearm and refuel.
You need a computer network with multiple computers and systems able to perform multiple different functions for that sort of thing
For a long time the military talked about C3IR... command, control, communications, intell, and recon... it was in the late 1980s they added the extra c... computers... to make it C4IR but it is a very very important addition that made a difference across the board in terms of everything.