For the time being, you can attempt to hack it in with the <details> and <summary> HTML tags. You’ll have to forego writing in Markdown if you do, though. That includes turning newlines into linebreaks, you have to write your own <p> and <br> tags. Extremely annoying.
Example 1
Example (click me)
Text that is hidden by default
<details><summary>Example (click me)</summary><blockquote><p>Text that is hidden by default</p></blockquote></details>
Example 2
Click to hide text
Text that is shown by default but can be collapsed
<details open><summary>Click to hide text</summary><blockquote><p>Text that is shown by default but can be collapsed</p></blockquote></details>