The OECD published the results of a massive survey of member countries some years ago, titled "Skills Matter" (https://www.oecd-ilibrary.org/education/skills-matter_978926...). The researchers defined 4 levels of technology proficiency, based on the types of tasks users can complete successfully. There was a very good summary published here (https://www.nngroup.com/articles/computer-skill-levels/) and excerpted below.

For each level, here’s the percentage of the population (averaged across the OECD countries) who performed at that level, as well as the report’s definition of the ability of people within that level:

“Below Level 1” = 14% of Adult Population

Being too polite to use a term like “level zero,” the OECD researchers refer to the lowest skill level as “below level 1.”

This is what people below level 1 can do: “Tasks are based on well-defined problems involving the use of only one function within a generic interface to meet one explicit criterion without any categorical or inferential reasoning, or transforming of information. Few steps are required and no sub-goal has to be generated.”

An example of task at this level is “Delete this email message” in an email app.

Level 1 = 29% of Adult Population

This is what level-1 people can do: “Tasks typically require the use of widely available and familiar technology applications, such as email software or a web browser. There is little or no navigation required to access the information or commands required to solve the problem. The problem may be solved regardless of the respondent’s awareness and use of specific tools and functions (e.g. a sort function). The tasks involve few steps and a minimal number of operators. At the cognitive level, the respondent can readily infer the goal from the task statement; problem resolution requires the respondent to apply explicit criteria; and there are few monitoring demands (e.g. the respondent does not have to check whether he or she has used the appropriate procedure or made progress towards the solution). Identifying content and operators can be done through simple match. Only simple forms of reasoning, such as assigning items to categories, are required; there is no need to contrast or integrate information.”

The reply-to-all task described above requires level-1 skills. Another example of level-1 task is “Find all emails from John Smith.”
Level 2 = 26% of Adult Population

This is what level-2 people can do: “At this level, tasks typically require the use of both generic and more specific technology applications. For instance, the respondent may have to make use of a novel online form. Some navigation across pages and applications is required to solve the problem. The use of tools (e.g. a sort function) can facilitate the resolution of the problem. The task may involve multiple steps and operators. The goal of the problem may have to be defined by the respondent, though the criteria to be met are explicit. There are higher monitoring demands. Some unexpected outcomes or impasses may appear. The task may require evaluating the relevance of a set of items to discard distractors. Some integration and inferential reasoning may be needed.”

An example of level-2 task is “You want to find a sustainability-related document that was sent to you by John Smith in October last year.”
Level 3 = 5% of Adult Population

This is what this most-skilled group of people can do: “At this level, tasks typically require the use of both generic and more specific technology applications. Some navigation across pages and applications is required to solve the problem. The use of tools (e.g. a sort function) is required to make progress towards the solution. The task may involve multiple steps and operators. The goal of the problem may have to be defined by the respondent, and the criteria to be met may or may not be explicit. There are typically high monitoring demands. Unexpected outcomes and impasses are likely to occur. The task may require evaluating the relevance and reliability of information in order to discard distractors. Integration and inferential reasoning may be needed to a large extent.”

The meeting room task described above requires level-3 skills. Another example of level-3 task is “You want to know what percentage of the emails sent by John Smith last month were about sustainability.”
Can’t Use Computers = 26% of Adult Population

The numbers for the 4 skill levels don’t sum to 100% because a large proportion of the respondents never attempted the tasks, being unable to use computers. In total, across the OECD countries, 26% of adults were unable to use a computer.

That one quarter of the population can’t use a computer at all is the most serious element of the digital divide. To a great extent, this problem is caused by computers still being much too complicated for many people.

Let that phrase sink in: across the OECD countries, 26% of adults were unable to use a computer. In some countries like Japan, the number is even higher (about 1/3 of Japan's population can't use computers, which may reflect the aging population, poor interface design, or some other factor.)

These data were based on surveys from 2011 through 2015, and if TFA is correct about the usability trends, surely it's gotten worse.