The most commonly used Markdown variants (Github, So, Reddit, probably Slack) support tables, they've been in use for years and work well. I can't believe Commonmark can't just standardise what's already being used by the majority of users.
Some of the suggestions in the discussion thread[0] don't pay any attention to this, and if they agree on something that nobody uses it'll just be ignored.
[0] https://talk.commonmark.org/t/tables-in-pure-markdown/81/29
Some other, non-standard variants of Markdown also handle tables via a syntax which resembles ASCII art. And while that format does do an excellent job of adhering to Markdown's ethos of remaining readable in plain text; I usually prefer to use HTML fallback anyway as it's easier to maintain.