Problem
The Thanks extension overly enforces partial blocks:
https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/Thanks/+/f3d1aac4db93679d400c20b2274515d94913c718/includes/ApiThank.php#16
This prevents users who are partially blocked on a page or a namespace from sending a Thanks to anyone, anywhere on the wiki.
Solution
If a Thank is for a revision, then PermissionManager::isBlockedFrom() should be used to determine if the user is blocked from the Title of that revision.
If a Thank is for a non-revision (a log entry), then Block::isSitewide() should be used to determine if the user is blocked from the entire site, which would prevent the use from sending any Thanks.