Those yellow "exit-only" designations on OSBs

First of all, an OSB is an overhead sign bridge - one of those big green things that tell you where you need to go, identify the highway you're on or let you know what lane an exit might be in. They may be considered the most important navigation tool a driver has, and paying attention to them is critical as they relay some really, really important information.
One thing an OSB might have is a yellow "exit only" designation. This marks a lane that's for exiting only, alerting a driver the lane under the yellow tab on the sign is about to exit the highway.
A short while back a reader named Juan reached out and asked about these yellow exit only designations. Here's Juan's email verbatim:

I'm curious about the usage of yellow "exit only" designations on overhead signs on local highways. I think they're generally very helpful, since they allow observant drivers to plan ahead as they approach the exits.
I've noticed one or two locations, though, where the yellow "exit only" signs are absent. Most recently, I was entering a congested northbound 281 from San Pedro Ave, planning to exit on Bitters Rd. The Wurzbach Pkwy exit sign didn't have one of these yellow "exit only" markers attached, and I didn't realize until too late that my lane was being forced to exit.
Fortunately, of course, the Wurzbach Parkway exit was perfectly acceptable for me, since I was just going to Bitters Rd anyway. But I can imagine it being more of an inconvenience for someone whose destination was farther north.
Is there some nuanced reasoning for these signs being present at most "exit only" locations but not at all of them? Or is this simply an oversight?

Juan, thanks for your patience in us getting an answer to this. We've visited with our traffic operations engineers at length to find the right answer and make sure you get the question addressed as completely as possible.
Most of the answer is found in our Manual for Uniform Traffic Control Devices, which can be found online here. There are some situational issues here the MUTCD doesn't address, though, so we'll take a crack at it.

Use of the Exit Only panel
First of all, let's define when we are supposed to be using those Exit Only tabs on the OSBs. The MUTCD tells us they're only to be used when we have a lane drop on the freeway through lanes. Because auxiliary lanes aren't freeway through lanes - they tie an entrance ramp to an adjacent exit ramp - we haven't typically used the Exit Only tabs at these locations, and these make up the majority of the lanes that might qualify as "exit only" locations across the area.

Auxiliary lanes

Auxiliary lanes are those temporary lanes you have linking an entrance ramp and the next exit ramp. They allow entering traffic an opportunity to accelerate to highway speeds and comfortably merge with freeway traffic. They're a fairly simple way to address some of our congestion issues in targeted locations - like on I-35 in the Schertz-Selma area, where we added auxiliary lanes and adjusted some of the entrance and exit ramps a few years ago.
Auxiliary lanes are typically striped differently than normal through lanes, using extra wide lines and the large dashes. We'll also use, where needed, arrows on the road surface to ensure drivers are aware they're not in a through lane.
We do have guidance in one of our handbooks telling us when an auxiliary lane is longer than 2,000 feet we should sign it as though it's a through lane dropping. We try to follow that the best we can. Most auxiliary lanes are much shorter than this, though, and do not require the additional marking with the yellow tab on the OSBs.

Some inconsistencies
We know of some areas where we've got an unnecessary Exit Only panel, and others where we should but don't. For example, on northbound US 281 between San Pedro and Bitters, the auxiliary lane is about 2,500 feet. We should have that Exit Only panel but we don't - likely an oversight during the last construction project we had.
Our traffic operations team looks for and keeps tabs on these locations to make sure the issue is addressed during our next construction project in the area. Those OSBs are crazy expensive (can cost more than $50,000) and we don't like to pull them down without already having the work planned through other projects. So, where we know we have work going on in an area, we'll take a survey of our signs and make sure to include any needed upgrades or revisions.