Those bumpers you refer to are the posts at the entrances to the outlanes, which are the channels at the far left and right away from the flippers, where you can lose a ball. Those posts are now almost universally adjustable, for a wider gap to drain more balls or narrow to keep them in play longer.

There's a few games with a (computer controlled) block for the center drain too. But mostly, early drains have been addressed by kick-back and ball saver. I don't know exactly when ball-saver came around; my Bride of Pinbot (1991) doesn't have it, although it does light an extra ball for free on your last ball if you ran through your first several balls (defaults for three ball play, but configurable) too fast (configurable by the operator, default may be 60 seconds?), but my uncle's Indiana Jones (1993) had it (Indy: Don't touch anything) and the interwebs say The Addams family (1992) had it too, but I didn't play that enough to have the ball saver phrase stuck in my brain.

Ball saver was invented specifically by Terminator 2 (1991 after Bride), because the plunger sends the ball into the center of the play area, where it could drop down the middle before you got any chance to do anything with it.

Addams supports ball saver in software, but it's not on by default factory settings, as it is for almost all later games, so you only get it if the operator enables it in the menu. Designer Pat Lawlor didn't like the ball saver crutch and also tried to minimize its use in Twilight Zone (1993), though after that it became fully standard.

Addams does have a different form of ball saver like this, though. Almost all games dating back to the early 80's will give you the ball back if it drains without ever hitting any playfield switch at all. This happens because the machine can't distinguish between this case and the ball failing to eject out of the trough to the plunger in the first place, so it errs on the side of letting you play again.

For Addams (and other games but most notable for Addams), exploiting that no-switch-drain became a strategic point: deliberately plunge softly so the ball will get to the flippers without hitting any switch, and if you fail to trap and gain control at the flipper, you'll get it back.

What you mention for Bride is called a "pity extra ball", lighting one for you if you did very badly before starting ball 3 (usually determined by score, not play time.) A fair number of Williams machines had that in some form, and occasionally later Sterns do too. It's functionally the same as an extra ball awarded by any other means, and nothing to do with any kickback or ball saver.

Kickback at the bottom of an outlane dates back to at least Firepower (1980), which has target banks at an angle that will often rebound the ball into the left outlane. I'm not sure if anything earlier than Firepower had that.

Bride of Pinbot is a sadistically difficult game.

It can be, depends a lot on how steep it is and if the flippers are good. Weak center ramp shots can often go back straight down the middle. If you get the ramps down, though, you can just spend all day hitting the center ramp, or go up the left ramp to advance the story (that said, I can never get the billions without cheating, and my machine has had some weird issues for a while, going to try a new cpu board and hope)

Pinbot isn't exactly easy either.

Agree 100%. I am OK at pinball but it's depressing to have the balls drain in the first minute on a new game.