Problem/Motivation
It is impossible to disable a block from the block configuration edit page; you need to go to the Block layout listing (/admin/structure/block) to select "Disable" from the dropdown options there.
Anything that affects a block's configuration should be doable from the individual block Configure block page (/admin/structure/block/manage/exampleblock)— this would be anybody's expectation. And indeed this is the case for translating the block (available as the tab "Translate block") and removing the block (available as the button "Remove block").
Proposed resolution
Add a button "Disable block" between "Save block" and "Remove block" or turn "Remove block" into a dropdown button with "Disable block" and "Remove block".
Remaining tasks
This problem shows up in contrib fairly often too; there may be a deeper layer in core than the Block module that this can be fixed. In any case the solution should be documented.
User interface changes
A new button is added (either on its own or as part of a dropdown)
A "This block is enabled" boolean toggle is also an option but this makes more sense as a button.
API changes
None.
Data model changes
None.
Release notes snippet
Blocks can now be disabled form individual "Configure block" pages. If your module provides configuration that can be disabled, not only deleted, it is recommended to follow the same practice.