Lauri shared this feedback with me. I believe the blocks enabled for Canvas is configuration that either the default recipe or the site template controls.
This should be fixed by Canvas beta (or might need CMS update?) but we should make sure that blocks that are absolutely irrelevant to be used in Canvas are not visible in the library. For example, the announcement feed block should not be available.
Install Byte. Blocks that are "internal" are exposed.
Review which blocks are for the frontend, keep only those enabled.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.