SPOILERS! Mysteries and Conspiracies of Pokemon

[Hugin]

I'm shocked we never got a generic "STAB Beam" of some sort that just matches your primary type and has ~80 BP. We went like 3 gens where Hidden Power was the best STAB some types got and there's still occasional weird mons that are desperate for at least one of their two STABs(not to mention how half the type shifting mons end up with an equivalent of that ANYWAY), but it's a gap left unfilled and feels like a simple QOL fix that they would have done accidentally at some point.

 

[pika pal]

Obviously this doesn't explain bug being SE vs poison. But I think there was likely an intention to add more bug type moves (and probably more moves of other under represented types) early in development that got dropped at some point. Or maybe like other RPGs, very early in development pokemon used generic attacks and always did damage that matched their type and then got changed to type based moves at some point and they forgot to actually make all the moves.

My guess is before Bug Type moves failed to materialize in some notable way, Bug Moves might have been a way to help the "crutch character" design of early Bugs like Butterfree and Beedrill. Against the as-mentioned Generic Poison "Evil" Pokemon, your Bug types could hit harder while you'e getting other guys caught up (and the dual-SE relation might even result in stuff like Bugs heavily hitting Zubat or Ekans or such, then being KO'd so the next mon takes all the EXP off the weakened opponent), akin to FE Pre-Promotes. They'd fall off when the Rockets got more varied rosters in theory and you had more trained mons with type coverage and better late game stats then (barring Scyther and Pinsir who also come late game as single stages).

One thing this makes me think of is how Erika as the 4th Gym Leader would about coincide with the "last hurrah" for Bugs, and if Poison was resistant to Bug as it is now, they wouldn't beat her Grass Types, one of the other few advantages the type had in Gen 1.

 

[Ironmage]

Scyther and Pinsir in gen 1 feel like they were hit pretty hard from a lack of focus on STAB moves. They were thematically tied to Slash and ViceGrip respectively, both Normal moves (Slash gets used elsewhere, I imagine ViceGrip wasn't changed for parity reasons). Since the only two Bug mons that are supposed to stick around when non-Normal attacks become commonplace (quite a while in gen 1) weren't supposed to be using Bug moves as their primary options anyway, I could see how Bug got a low priority when it came to filling out options.

Scyther now has Aerial Ace as a good thematic STAB, but non-Mega Pinsir still feels like it struggles a bit in this regard.

 

[R_N]

Scyther and Pinsir in gen 1 feel like they were hit pretty hard from a lack of focus on STAB moves. They were thematically tied to Slash and ViceGrip respectively, both Normal moves (Slash gets used elsewhere, I imagine ViceGrip wasn't changed for parity reasons). Since the only two Bug mons that are supposed to stick around when non-Normal attacks become commonplace (quite a while in gen 1) weren't supposed to be using Bug moves as their primary options anyway, I could see how Bug got a low priority when it came to filling out options.

Scyther now has Aerial Ace as a good thematic STAB, but non-Mega Pinsir still feels like it struggles a bit in this regard.

I'd say X-Scissor works thematically for Pinsir, the cross-section of the horns and all