There are any number of reasons why they weren't seen but I'm just saying I far preferred their not showing up until ROTJ to be a unspoken sign that the Alliance was gaining against the Empire, on the political and technological front. I think A-wings (okay they have semi-bubble canopies) and B-wings would've been incredibly useful during the battle of Yavin, intercepting TIEs before they could attack the other fighters making their runs and minimally, the B-wings would've surely been no bigger or slower than the lowly Y-wings.
Of course, I wonder if these specialized roles that had been established for these fighters even exist anymore. Was that a product of the EU?
I haven't seen anyone commnt to this, so I will. Yeah, those roles are all from the former EU. I honestly have no idea where the "Y-Wings are slow and lumbering" thing originated, cuz that's not how they're portrayed in ANH. Red Leader says accelerate to attack speed (which I assume means something at or near full thrust) and the Y-Wings are moving just as fast as the X-Wings. From what we saw of them in the trench and, later, the reactor run on the DSII, their maneuverability is comparable, too.*
[*As an aside, I was re-re-re-watching the space battle in ROTJ to see what I could glean for a full post-ANH Red Squadron and cripes it's a mess. The groups/squadrons/whatever don't seem to be sorted by ship type, if dialogue is any indicator. The guy who calls in as Gray Leader (in a Y-Wing) is called Red Two by Wedge later. Assuming he's Red Two, the person who responds when Wedge says "Red Two, Red Three, let's close it up", and is thus presumably Red Three, is an A-Wing pilot. And when the shields drop and Lando orders all fighters of Red and Gold Groups to follow him in, we see Wedge lead the way in his X-Wing, then an A-Wing (presumably Red Three, above, as Two and Three seem to stick pretty close to him during the battle), then the Falcon
, then another A-Wing (with the old "Copy, Gold Leader" guy flying it), then a Y-Wing (presuming Red Two, for reasons above), then another Red Squadron X-Wing. Five fighters, all seeming to be of Red Group, are all that fly in with Lando. No one heads in a bit later and chases the TIEs, no commentary about that being all that are left. Just... kinda weird.]
Do we have anything in canon that says what astromechs do aboard fighters besides in-flight repair and quasi-copilot duties? Astromechs as navigators is pure EU, as far as I know, so no canon need for them to make hyperjumps... For all we know, A-Wings could be decades old, having been supplanted by the AOTC and TCW
Aethersprite interceptors, and the later V-Wings. I can see a steady evolution through those -- no astromech or hyperdrive on the A-Wing, astromech physically incorporated into the too-thin-for-a-socket early-model
Aethersprite and an external hyperdrive ring, relocated astromech allowing a full socket and integrated hyperdrive on the later version, then a purpose-built astromech relocated aft of the cockpit and augmented shielding/radiator wings/panels presumably allowing the bigger and more-powerful weaponry on the V-Wing.
The same engineers and techs who kept Z-95s and Y-Wings flying post-Clone Wars probably built the newer and more compact hyperdrives into the A-Wings they were keeping flying. We know from real-world examples like the A-10 and B-52 that newer tech developed for other applications can keep older designs viable long after they were expected to be obsolete.
And with the EU gone and Rebels resetting their debut date, we know practically nothing about the B-Wing, other than from its sheer mass it's almost a gunboat rather than a fighter.
--Jonah