Does Slack support Markdown tables?

Slack does not support rendering of tables so this markup will not work.

You have two alternatives:

• You can use fields, which will be rendered as 2 columns on most devices. See fields in layout blocks.
• You can convert your table into an image (outside Slack) and
  attach the image to your message.

No, in fact, Slack doesn't support Markdown in messages¹ at all. It uses its own similar-at-a-glance format called mrkdwn which has some notable differences with Markdown:

• In Markdown, both * and _ are used for emphasis
• In Markdown, both ** and __ are used for bold
• In mrkdwn * is used for bold and _ is used for emphasis
• Markdown has no syntax for strikethrough (though some implementations have added it, e.g. in GFM which uses ~~) but mrkdwn uses ~ for strikethrough
• Link syntax is very different
• mrkdwn doesn't support headings
• Probably more

Don't expect arbitrary Markdown² to work in Slack messages.

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¹Slack does support Markdown in posts which can be created using the files.upload API endpoint setting filetype to post.

²Note that tables aren't supported in regular Markdown either. Like strikethrough, some implementations have added these.

Add ``` before and after your table in your str message. it will convert in code block in slack meassge.