Issues can be created via https://github.com/SlimeKnights/TinkersConstruct/issues/new/, but this bypasses issue templates. Instead, the 'new issue' button leads to this screen:

Github bug, nothing we can do about it. See community/community#4530, related to community/community#4696.

Closing as this is on Github to fix.

Edit: misread the issue. Github changed an optional field in the issue form to required and as a result just hides all our issue templates. They are now updated.