There's a todo in block.html.twig to the effect that we should remove the block ID attribute since more than one block can appear on a page. However, it's still useful to be able to target blocks independently. I propose using the attributes.id concatenated with the block_id variable (since it's a counter).
Although the docs do say attribute.id is guaranteed unique. So not sure there.
Fixed
8.0 ⚰️
Last updated
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I am moving this issue back to Drupal core from the (now deleted) sandbox project used for the development of the theme engine that uses Twig.
I apologize for bumping this issue.
Drupal core as of 10.2 (i.e., the 11.x branch) now has clean_unique_id → Twig filter, thanks to ✨ Add a new clean_unique_id Twig filter for Html::getUniqueId Fixed . Which might make change the proposed resolution and patches.