One way to think of it is that you're not always being paid to be doing something specific, but rather to be available on short (e.g. 15 minutes or less) notice.
In development at least, most of the times I hear this, the "availability" is only needed for random unplanned meetings from useless managers that can't actually plan or work asynchronously.
The organization as a whole or individual managers can't do their jobs well and they make the people that actually get shit done pay for it by "standing by" for no good reason.
Ironically, they also completely ruin productivity with these interruptions.
Really? Because it's pretty routine for me to get pinged on IRC/slack/whatever by another engineer who's looking for help with something. Obviously I set the expectation that I'm not going to be able to instantly reply to every ping so some things are going to have to be async, but there's definitely a lot more than just managers that could ping me.
The vast majority of time in my career, if you have a minimum amount of decent planning, you won't get blocked because you can't talk to another person immediately. And on healthy organizations, even if you do get blocked, you can simply work on something else while that particular task is blocked.
If you really scrutinize all these availability claims, they pretty much all come down to lack of planning.
Planning for blockers, planning for communication gaps, planning for maximizing asynchronous work, planning for decisions that require specific people, etc.
So many people are horrified by the word "blocked" when it's most times an artificial block created by their own systems. If I can't move forward on something, I'll start on something else and come back to the other one later.
We are all intentional about _over communicating_ what we are working on and where we will need support from from each other. Everything else pretty much just flows naturally from that. There are (rare) times were I cannot progress on something that day because I need feedback from someone who will not be online for 10 more hours, but it is just a built-in assumption that this will happen and I always have other work queued up that I can do if I get stuck....
That's why when we were all colocated in one building, I was logged out. I made it clear they were welcome to drop by my cube whenever they wanted, but I would not be available by IM. It cut the interrupts by half, easily. People use you as a crutch. When you raise the cost even minimally, they are suddenly willing to solve their own problems.
That's the one aspect that COVID destroyed. Now I have to be online as people can't just walk to my cube.
Logging out of IM when busy is a simpler skill :-)